Mamiya 645AF Reliability

So I'm currently shooting with the older Pentax 645, but am thinking about switching to a Mamiya 645AF, mainly because of the backs. 1) I'd rather be able to switch mid-roll, 2) The Pentax inserts pose serious problems for shooting IR if I don't want to bring a changing bag. 3) Polaroid backs are much, much cheaper for the Mamiya. I have shot a bit with the Mamiya, and it's very nice, but it feels a bit plasticky compared to my tank-like pentax.

So I was wondering if anyone could comment on the reliability/sturdiness of the Mamiya body. I don't really shoot any AF. I just happen to be able to buy a 645AF body cheaper than a 645 Pro and an AE finder.

Where can you buy the AF for cheaper? If you don't want it, I might. The Pro and Super that I have are pretty solid cameras, but I've never tried IR film in the backs so I can't comment on that. More solid is the 1000s but then you lose the film back. I guess it comes down to if you are putting your cameras into situations where they may get physical damage. The pro and super are not as strong as the old 1000s, just a fact of life that you give up when you go to the removeable film back and plastic covering.

I assume the 645AF is pretty close to the Super and Pro in build, but I have never used one so it is just speculation.