If I go with a Mamiya 645 Super is that easily serviceable?
The 645 Super had a relatively short production life. One of the major reasons for that apparently was that it, and the 645 Pro and 645 Pro Tl that superseded it, were marketed to wedding photographers who frequently used the cameras with motor drives. The gearing in the Super was/is a weaker point, and heavy users who employed motor drives ran into problems. The 645 Pro incorporated more robust gearing, added a self timer, deleted the single "no battery required" shutter speed, and had a much longer production life.
If you come upon a 645 Super that wasn't often used with a motor drive, it may very well serve you well for a long time. But as is the case with all three of the Super, Pro and Pro Tl versions, many of them have seen a lot of heavy use.
I had a cosmetically "interesting" Super for a few years, and it served me well. I added a 645 Pro, and it has been an excellent camera for many years.
The cameras were designed for the professional market, with the expectation that they would receive normal maintenance. Some of the parts were designed to be replaced when worn. That included mirror stop bumpers. Original replacement parts are no longer available, but new sources have arisen through the internet.
The Mamiya 645 line was produced for a very long time.
Roughly speaking, there are three stages - the early manual focus models with fixed backs (e.g. the 1000s), the intermediate manual focus bodies (Super, Pro and Pro Tl) with interchangeable backs plus the relatively light duty fixed back 645e, and the auto focus models with interchangeable backs that are partially compatible with the manual focus lenses. Some of the last auto-focus bodies could be used with some early digital backs as well.
The later auto-focus lenses can not be used with the manual focus bodies.
Many of the system accessories are compatible with many of the versions, but not universally. Many of the (probably too many) accessories I have are compatible with all or most of the manual focus bodies - particularly the lenses and film inserts. Some of the accessories I have only work with some of the versions - e.g. the connector for the accessory grip.