Consider the Rb67 Mamiyas
When I looked at what I wanted, the Mamiya RB67 Pro S was the ticket. The advantages are they are cheap, the negatives are the next thing to a 4X5 and you have a camera that can go digital if film ever becomes history. Also, you can put a Polaroid back on them. Things you can't do with a TLR. With some of the TLR's you are stuck with a very limited amount of lenses. If you are doing rapid changing of film, with the RB's you can have inserts that were previously loaded and just pop them in. This is true of the 645 AF's too. The reason that a lot of cameras get slower speeds over time I think is that they don't get used. Even if I don't use a camera for some time, I take it out and trip the shutter at least once a month. If you are using your Rollei enough, you shouldn't be having much problems with it getting slow shutter speeds and such.