It certainly makes sense for you to stick with Nikon since you've got a lot of lenses to use on it. Just be aware that, by design, via adapters both Canon and Sony are able to accommodate vintage manual focus Nikkors as well as other F-mount lenses. Sony can even accommodate some other brands' AF, although the adapters for those tend to be rather pricey. The adapters for manual focus, manual aperture lenses are inexpensive... often $25 to $40. So, you might have more choices than you think.
However, assuming you want to stick with a Nikon DSLR for the familiarity, brand loyalty and appreciation, plus the ability to use many of the old lenses without need for adapters... I'd recommend you use some of the comparison sites such as cameradecision.com to help you choose. There it's possible to see the specs of any two of the DSLRs you're considering side-by-side. For example, I set up to compare D750 and Df....
http://cameradecision.com/compare/Nikon-D750-vs-Nikon-Df and to compare the D810 and D850...
http://cameradecision.com/compare/Nikon-D850-vs-Nikon-D810It's also possible to do side by side comparison of the general specifications at some retailer websites, such as:
https://www.bhphotovideo.com/c/compare/Nikon_D850_vs_Nikon_D810_vs_Nikon_D750_vs_Nikon_Df/BHitems/1351688-REG_1062499-REG_1082599-REG_1013089-REG (Note: Double check info on these and all other websites... I've noticed occasional errors and omissions.)
Nikon themselves may offer similar comparisons, though I'm not familiar with their websites. I do know they consider the Df and D750 to be "enthusiast" level models and the D810 and D850 to be "professional" grade. One difference is that the pro models have higher top shutter speed and faster flash sync (1/8000 vs 1/4000 and 1/250 vs 1/200). I don't know for certain, but this also likely indicates some difference in the durability ratings of the camera's shutter and other mechanisms. Often pro grade are rated for more shutter actuations ("clicks") than enthusiast level cameras. That may be a consideration, because I bet you'll find you shoot a LOT more images with digital, than you ever did with film (I know I do!). Because of the "unlimited free" perception of shooting digitally, early digital cameras simply adopted shutters and other mechanisms from film cameras and those early digital sometimes had very short lifespans! All the manufacturers learned from their mistakes and developed more durable shutters and other mechanism but still use different grades depending upon the intended user, in order to be price competitive. A $1500 D610 with 1/4000 top speed likely has a lower grade shutter rated for a lot fewer clicks than a $2800 D810 with a 1/8000 top speed. (For example, Canon cameras that I use are rated for 200,000 clicks... while more entry-level/enthusiast models are rated for between 75,000 and 100,000 clicks. Some late 1990s digital with film camera shutters were only good for 20,000 or 30,00 clicks and their manufacturers ended up replacing shutters on some hard-used cameras once or twice under their 1 year warranty... which no doubt was costly!)
Coming from film-only, you are going to be absolutely delighted by some of the things you'll be able to do with digital such as shooting hundreds of images without reloading film, not having to continually buy film and pay to get it processed, getting immediate feedback anytime after a shot via image playback and the histogram, ability to change ISO from shot to shot whenever needed without having to change film... not to mention FAR higher usable ISO than was ever possible with film... and far less need for filters (thanks to adjustable white balance and post-processing software that can emulate many filters or even do a better job than some)... and much more.
In spite of the advantages of digital, I can sympathize with your appreciation for the film SLR. I have a large number of vintage cameras, including many SLRs and 10 Nikon (some the same as you, though most older). I love 'em, but it's a whole new ball game shooting digital! While you might be tempted by the Df's "retro" design and similarity to film models that you know and love, you may miss out on some important and convenient features that are offered by modern design DSLRs... such as an articulated LCD screen that makes easier overhead or low down shooting, WiFi connectivity, video, an anti-flicker mode that solves much of the exposure problem under fluorescent lighting, Touch Screen controls and much more.
And while the Df certainly is a neat camera, at 16MP it's now somewhat "antiquated" and low resolution for a full frame DSLR. This may be enough for your purposes... or not. Really depends upon what you want to shoot and how you want to use your images.
In fact, even the 24MP of the D750 is somewhat on the low side among modern full frame DSLRs... many of which now have upwards of 30MP (D810: 36MP) and are approaching 50MP (D850: 46MP).
Ultra high speed, professional "sports/photojournalism" cameras such as the D4s and D5 tend to be lower resolution (21MP, in these cases). This is done deliberately because it's sufficient for their intended purpose (such as images for newspapers, magazines and their respective websites) and allows their very fast frame rate (D5 can shoot 200 consecutive images at 12 frames per second) as well as extremely high ISOs (D5 has a native ISO range of 100-102400, extendable to as high as ISO 3280000.... yes that's correct, ISO
three million!.... which may or may not be actually usable, depending upon the purposes).
While the control layout and look of the models other than Df may feel unfamiliar at first, it sounds as if you have some experience and I bet you could learn to use just about any of them in relatively short time. So do a lot of comparisons before you decide... The Df, while a cool retro camera, is now a 4 year old model (late 2013), which is a full generation or two among digital cameras. Model life spans of DSLRs are nowhere near as long as film SLR's were... Although not changing nearly as rapidly as in the late 1990s and early 2000s, digital technology is still making steady advances that keep us all "upgrading" long before our cameras actually wear out! The D750 is about a year younger and the D810 was announced the same year (2104). The D850 is the newest, released just last Fall, so three to four years newer and Nikon's "latest and greatest".
There are "guide books" written for most camera models, which go into great detail helping folks learn to use their camera... sort of complementing the user manual the manufacturer provided. I recommend those and always buy one for any new model I acquire, just to keep my learning curve to a minimum and to make sure I don't overlook something I'd find useful. Search in "books" on Amazon.com for any particular camera model.... I'm sure you'll find a selection for any of these Nikons. David Busch, Charlotte Lowrie, Doug Klostermann and David Taylor are authors whose guide books I've found very useful, though I'm sure there are some others writing excellent guides (buyer feedback on Amazon may be useful).
Whatever you get, I'd suggest you consider getting it in kit with a modern AF-S lens such as the Nikkor 24-120mm, for general "walk around" purposes. It's often the best deal to buy a lens this way, nicely discounted from buying separately. Lenses have developed optically in recent years along with the DSLRs they're mostly used upon. Great as some old lenses are, digital cameras are more susceptible to flare, so new coatings and other tweaks have been done to optimize modern lenses on digital cameras. The AF-S system might out-perform the older AF "screw driver" you're familiar with, too (though all the cameras under consideration can accommodate both).
Finally, you might want to consider renting before buying... take a test drive and learn a bit more about any particular camera, finding out if you can get comfortable with it.
Have fun shopping!