I will add my two cents since I was in your boat around 5 years ago.
Mamiya 645 Pro......the TL is nicer for more modern options such as finding used Metz 45 or 60 flash you can use TTL with, but my Pro with even first gen CT60-1 flash gets great flash photos. I looked at every other option like you did but went with Mamiya mainly due to slightly better build factor and there is always a Mamiya 645 something or other available somewhere.
I believe you can still get the Pro TL serviced by Mamiya still today, with all the others, you may be able to get service, but parts will be limited. That said, I don't know how hard by Pro was used, but I still have no need nor concern about a CLA or any other service issue with mine.
One more thing......I have not been actively looking for some time, but when I did there always seemed to be at least one accessory or another available for the Mamiya 645 system. There is a huge chart somewhere that displayed all the extras they made for that system at mamiya's website or in the 645 manual somewhere......I found virtually every one of those "extras" for sale for my 645 years back.....like 4 years back. After you get the regular 3 lens setup you will get the fever and start wanting those little extras for your setup.....another reason I voted for Mamiya vs. the other guys.........and the other guy's cameras are all noteworthy performers too.