"IMHO, that one should start with a system that you can at least change lenses. Having a fixed lens medium format is nice, but sometimes, you want to changes lenses and you can't do that with a TLR or something like that."
There is something to that. But if one is just making the jump to larger formats, it is worthwhile to consider the construction of a frame once again. We are all too wrapped up in different lenses and zooms and this that and the other we sometimes forget that the photograph is only as good as we visualize it. Once in a while you have to get up on something, or run back. If the thing doesnt appear quite "right", then what would make the shot more interesting with the perspective that is available?
I love gizmos and gadgets. I got them for 35, 6x6, 4x5, and God help me am collecting for when I take the 8x10 plunge. But any and all of them can take away from the simple task of thinking through a photo with bare resources. A new "old" TLR has reminded me of that, and I am finding that there are a lot more "keeper" shots in the 12 exposures than the 36 that travels along with it.