I am just a hobbyist and have only MF experience with Hasselblad "V" systems. (I have 5 bodies and about a dozen lenses, film and digital backs, etc.) One potential advantage of 'blads is that, at least in the US, they still have excellent factory support for their older systems. They recently ran a "special" for CLA and service and I sent multiple lenses, film backs, bodies, etc. for service and repair. Turn around time was about two weeks and the cost, I thought, was reasonable.
KEH.con is an excellent resource for seeing what is on the used MF market and gives a starting point for pricing. I don't know about Mamiya, Pentax, etc. but buying used "V" series stuff over the past 5-7 years, the prices have remained stable if not increased somewhat; so if you decide to get out of the MF game in a few years, baring economic collapse, you are unlike to sustain a significant loss on resale. As an example, I bought a EX+ 503CW body from KEH about 6 years ago for about $900, similar bodies are listing for $900-1,200 now. Bottom line is buy latest and best condition within your budget.
Just a quick look at KEH, you could get a EX 503CW body, 80mm CF lens and late model A12 back for about $2k USD. If you have some flexibility in your budget, I would seriously consider getting a 203 FE body as the starting point for your MF system.
Steve