I built my last two computers and I recommend it; if your son-in-law is willing to tackle the job (assuming you don't want to do so), it is easy to build a machine that will give you the capability to edit photos and videos for years to come. The process is fairly straight forward. (All of this is Windows--I don't know if one can build a MAC or not, but I personally don't like Apple's proprietary-only equipment.)

I did try to order what I wanted from HP and others--I simply could not find the power, memory, and options I wanted (I could not customize the systems to give me what I wanted)--but it sounds like the builders others recommended might do the job for you.

First, you have to decide on the basic structure of your computer: I wanted maximum storage with protection from disk drive failure, so I planned for two two-terabyte mirrored (RAID 1) sets (four 2-TB drives--I probably would go with 4-TB drives now); I wanted a fast system with the capability to edit video without choking (my previous computer would load Adobe Premier Elements and the videos, but then freeze); and I wanted video cards that would provide fast, accurate renditions. What that meant was that I built what are essentially gaming computers without any intention of using them for games (I also have not used overclocking, although both my computers are designed for it). I also wanted four monitors (I do all of my work on my computers, in addition to my photography).

The number of drives, the amount of memory, and the number of video cards led me to the motherboard that would accommodate it all. My second build was after I had trouble with the first (that I later traced to two bad video cards, which I didn't think possible at first--replacing them made the first system run great). The first time I ever saw all six cores maxed out was when I edited and converted to DVD format about an hour's worth of video from a Canon 5D Mark III. I would not settle for less than 64 Gb of RAM in a new system now (my two are 24 Gb and 32 Gb).

So, more to the point: Get the fastest, biggest processor your budget allows, preferably one of the new Intel 8-core CPUs (or more--but a six-core CPU would work, too), matched to the appropriate motherboard (ASUS ROG boards are attractive options, because they are overbuilt for gaming, have sufficient slots to have multiple video cards, have the capacity for enough hard drives, and will last); get 64 GB (or 128) of RAM, also matched to the motherboard--it is not necessary to have the fastest memory, but lots of it is essential to photo work without interminable waits (I think the suggestions for 8 Gb or 16 Gb are outdated--more memory means more responsive operation); at least four hard drives, in two mirrored RAID 1 arrays (Windows 10 RAID is much simpler than hardware RAID, and much less finicky)--the bigger the better--one pair for Windows and all programs you will use, and one pair for your data, including your photos. (You should plan on checking your disks through Windows Disk Management regularly to make sure your paired drives stay "healthy" and synced.) You still need to have a separate set of external backup drives (USB 3.0 or faster), including one or more that you take to another location, if at all possible--the RAID arrays just provide insurance against internal failure, they do nothing to protect your pictures or anything else in the event of a theft or a fire or other destructive event. It may be that you can run the number of monitors you elect to use (fast IPS--look for IPS in the monitor description) on only one video card--when I built, I had to have two to run four monitors. If you try using at least two monitors you will never go back to a single one (just one example: being able to watch an on-line tutorial while using your editing software to follow along is much more conducive to learning than trying to work on a single screen). A 1500-watt modular power supply should run this without problem (1200 might, but add your system's power requirements and provide an adequate reserve to be sure there are no issues).

Nothing less than a full ATX tower will hold all of this stuff easily, and you should ensure you have plenty of cooling with whatever system you get.

Software: there are lots of recommendations in this forum, including a number of freeware programs. First, you have to have an operating system--Windows 10 Professional is the only way to go, for the maximum flexibility and power.

For digital photography, I have always used Adobe Photoshop Elements and Premier Elements (I am trying to become more adept with both), and I have Lightroom (that I have not used much), but there are real advantages to the subscription (Adobe Creative Cloud, I believe, which includes the full Photoshop, plus Lightroom and other programs, all up to date all the time)--I just have always preferred owning my software instead of subscribing. Irfanview is shareware/freeware and is really good for a lot of quick actions. I also use the software from my camera maker (Canon) for its noise-reduction for RAW images, and I think that Nikon has software for its RAW images, too (but I know nothing about it except for comments from others). Of course, you have to learn to use all of the software, and the complexity means you really need to be taught how to operate it all (there are lots of online tutorials and courses--Google your way to one you like), which can end up being more software. There are all kinds of specialty programs available--I have never tried any of them--and there are all sorts of add-ins for Photoshop (and Lightroom, I believe).

I am limiting my software recommendations to photography--I assume you have preferences for your office and email programs already.

I know the computer system I recommend is pretty complex and expensive, and that some will think it is overkill. But Adobe programs use multi-threading to make its programs more adept and powerful, and large picture files take lots of memory and storage to handle--the current Canon has a thirty-megapixel sensor and I think the large Nikon is thirty-six, and there are larger. A more powerful computer can really speed up editing, so you can spend more time shooting without dreading the time you will have to invest in post processing.

Good luck with your retirement, your new computer, and your renewed interest in photography.