where do you start...
the sills and front of the sills in the wheel arch (lowest point beheind the front wheel is the) first place to go.... next would be the rear subframe mounting points (they are visable from the rear wheel arch, infront of the wheel (stupid place to put them)...
Then check the back seat bins (underneath the cardboard insert - nobody checks there) If the car's had leaky windows, this is where the water ends up... it rots through the bottom of the bin, into the sills and out through the easiest point of exit....this being the rear subframe mounts
check around the front wishscreen seals looking for bulges (usually under the lower edge or bottom corners of the windscreen. the bottom corners are a nightmare to replace and mean the only proper way of doing it is to get rid of the wings at the same time.....avoid anything with a holy scuttle (panel at the base of the windscreen).
the front wheel bearings are very easy to overload so give them a wobble (as with the rear) the mini is very easy to throw into a handbrake turn...thus knackering any bearings youve got there.
listen for squeeks in the front suspension... go slow 15mph over a speed bump if you hear squeeks or a rubbing sound then the front rubbers are knackered.
the Bottom of the doors are a pain and WILL rot, but the panels are cheap to re-skin as long as your frame is good
I'm in the process of doing one up so if you need any help then give me a shout
they're great cars!! I've got 1 and a half
cheers
dave