As said above, theminiforum is very good, plenty of knowledgeable helpful folk on there.
Yep, they rust. Lots. My '94 i'm doing at the moment was a very nice example, once one bleb appears though, there's others lurking. I had to do LOADS of metalwork to it and it looked a cracker. I'm still finding bits now - I thought i'd whip the front shock top mounts off to clean and paint them - there was nothing left behind! Check the seams around the bulkhead and inner wing brace panels for any signs of grot.
They used to be cheap - now they are not. Parts are readily available, most repro stuff needs a lot of work, genuine is mega pricey. £478 for the rear subframe. Genuine inner wing i've just bought to chop up to repair mine was £180
Minispares are probably the best supplier - they seem to put a lot of effort into reproducing stuff so it's decent quality.
I reckon with modern underseals and rust preventers, and a bit of care it should be possible to slow the inevitable creep of rust - i'm building mine as a daily so im hoping, anyway