I’m in the process of designing and CNCing a custom bezel, so I hear where you’re coming from.
I would strongly recommend you buy a watch or a set first. Miyota is a good choice, however if you’re looking to combine with pieces, Seiko has, literally about 100 times more selection. Specifically, the “SKX007” form factor. (DWC uses the Seiko “sea urchin” form factor, which is slightly larger. I believe that makes assembly easier for us, but swapping parts outside of DWC is more challenging.)
When it comes to custom designing a case, that’s probably the hardest part, and you’ll need the following dimensions;
Diameter & depth of the interior of the case. All of the Seiko “NH” model movements have the same dimensions. (I believe Miyota 8- and 9- series have consistent dimensions as well.)
You will need a stem tube, usually at the 3 o’clock position. Some move that to the 4 o’clock position (really, closer to 4.1 on Seiko). if you are doing a calendar this must be aligned correctly or the date window won’t line up. If you forego the calendar, you can put the stem at whatever clock position you like.
The face diameter; if you are NOT doing a chapter ring, your only consideration is you must have sufficient space for the hands to swing freely. Chapter rings vary in diameter based on the model. Note that if you’re doing a GMT watch, this will impact the minimum depth of your watch. Adding a tall crystal will add some headspace, but there’s a maximum amount you can add through that method.
The crystal diameter. You’re not manufacturing your own crystal, so you MUST match the size of an existing standard.
Bezel inner and outer diameter and depth. Only bother with this if you’re using a bezel.
Space and positioning of the lugs (to attach the band). Lugs are usually set 20mm or 22mm apart for men’s watches, but other settings aren’t uncommon. I’m not sure how they determine the positioning of the spring bar holes. Among seikos, there are the “fat springbars” of the SKX007 and the regular ones, which are different, and even within seikos springbar location is not consistent. If you’re doing a NATO fabric strap, your life is 1000% easier here.
(For reference, look at how many dimensions change between the SKX007 and the SKX013. Both are from the same company and look almost identical. But a lot of those exterior measurements shift. Modding the SRPD/SKX007 vs SKX013 - A Comparison – namokiMODS )
I think I hit the big points… Installing the movement is easy. Line up the hole in the case for the stem with the hole in the movement for the stem. Once it’s installed, setting is almost trivial. I recommend youtube videos.
This is a big project and I commend you for tackling it!! If I were to make a recommendation, you might look for an existing watch off the market and shave it down to size. Invicta is a popular, inexpensive brand that sells large cases. They standardize on the Seiko NH series of movements, so you can swap between movements, dials, hands, etc. consistently. My only parts concern with the Invicta would be the bezel compatibility. This would let you worry about designing the outside of the watch, while ensuring the inside dimensions will work perfectly.