I never owned an a900 but there seems to be a very dedicated following and love for it. In particular people rave about it's viewfinder and colors (which some prefer over any other Sony camera produced).

I've shot extensively with a Minolta 50mm 1.8 in A mount but I find my Sony Zeiss 50f1.4 on the camera much of the time these days. I love how it renders the image.

I still use aperture and shutter priority but manual mode with auto ISO certainly has its place. It's very handy when I want specific shutter speeds and apertures but still need to be able to respond to changing lighting. My Sony a99ii also allows exposure compensation in this mode as well as more and max ISOs.

There are a few Sony lenses which are primarily Minolta rebrands with updated coatings. There are also some real gems in the legacy Minolta stuff. I have the Sony version of the 500mm mirror lens (which is the only AF mirror lens ever made I believe) and mine is very sharp. It is a very long, very light lens with unique bokeh. The biggest issue I figured I'd that the lens hood is short and it's easy to end up with veiling haze.

While we're talking about Minolta lenses, here are a few more great ones: 85f1.4 (there are several good versions), 100f2(this one is fairly small as well), 135f2.8 (pretty good and really small), 200f2.8 (a truly beautiful and sharp lens), 80-200f2.8 APO (comparative in sharpness to modern 70-200s), 135 STF, etc.

The big question is how much money do you want to spend? You can by a fast, current generation Windows or MacBook which will be quick but at a price of $1k to $2k. Recently I've chosen to purchase a one or two generation old laptop with an i7 processor and dedicated graphics card, then increase the RAM and install an SSD. I end up with a pretty quick system for roughly $400 to $500.

I am not certain as I don't have any older MD mount lenses, but I believe the focus confirmation will work. However, as has been stated, with focus peaking and focus magnification, you can manual focus more accurately than the focus conformation. The older lenses can also be adapted to the A mount and I believe Minolta made their own adapter for the first few years. One thing to check out is if infinity focus is still available with the older, adapted lenses. If you really want to engage the experts, Dyxum.com is the repository for all things A mount.

I have and love the a99ii but if your primary purpose is to utilize pre AF Minolta lenses then the Sony mirrorless cameras will likely serve you better.

Assuming you're wanting to stay in the Nikon camp, I've heard very good things about the 7200 and D500 (which is a beast). AS a couple suggested, if you're looking outside of Nikon, take a look at the A77ii as it's a very quick camera for the money.

First, have a grand time with your new camera! Next, as others have said, as I progressed in my own work, I find I have spent more and more time focusing on what I don't want in the picture than I have my primary focal point.

For me it would be my Sony a99ii. At 42mp it has the resolution to shoot high res landscapes. At 12 frames per second, it is competitively fast with the very fastest. Its AF is competent, its feature set as a stills camera, complete. I don't shoot much video but it'll do 4k easily (FF or super 35) with some limitations, 120 FPS 1080.

I like the Manfrotto NeoTec monopod. It doesn't retract to be as small as some of the others but there's nothing anywhere close to as fast. To extend it just lift up and to put it back down squeeze the handgrip.

As several have mentioned, the crop sensor lenses (the DT line in Sony nomenclature) will fit on the A99ii without issue and the camera will switch into APSC mode automatically. Also, with the 42mp sensor, even in crop mode it'll still shoot 18mp pics. With respect to lenses, every Minolta A mount lens produced since 1985 will AF on the camera just fine and Minolta made some really special lenses (80-200f2.8, sharper than the current Tamron 70-200; 100f2.0, 135STF, 500mm Mirror, 200f2.8, etc).

It doesn't matter to me. I have a Sony a99ii so I see everything I need in the EVF before I press the button as do all using mirrorless. However, I also shot film for 20 years where the was no chimping and with I little practice, getting a proper exposure was fairly easy.

A really long lens, great high ISO performance and good autofocus can absolutely be had, but it won't be small or light. At some point something has to give. The RX10 line is pretty popular so without making some concession somewhere, you probably won't do better. One thing to think about: it's common for people who get shoot a 500 or 600mm lens to have really bad results until they get the hang of it.

There has been some good advice in this thread but I'll recommend working on technique and getting a longer lens. The 1/focal length is a very good rule of thumb, but with good technique it should be possible to go lower. I especially recommend being very careful how you squeeze the shutter button as it's very easy to pull the camera while doing it.

There other thing is 400ft is a long way to be with that lens.