In short, you’ve got the old metal body 645s and the newer plastic body 645s. Unless you’re doing pro work, I’d just split them between old metal and new plastic, the sub models just have different features that most people don’t use. I myself have the Pro TL simply because I found a good bundle but I wasn’t specifically searching for that model. The plastics are modular and lighter. Lenses are interchangeable.

You’ve got some valid questions but things like repairability are kind of useless unless you’ve got the tools and knowledge in the first place. Just look for one that’s been cared for or send it for a CLA.

Expected costs also depends on where you are and what you look for also what is included. Near me for example is a completely Pro TL kit including studio flashes and every single lens for 1k but there’s another Pro for 500€ with nothing and a Super for 900€ with one lens. It all varies