..corrosion in the wing spar area in the cabin I think is the critical spot.
You can see the spar easily by taking off the cover behind the front seats. The spar sits in front of your shins if you are sitting in the back seat.
Also have a good look along the dorsal spine underneath the aircraft and if possible look at the longerons in the tail cone area. Pay particular attention to anywhere that is spot riveted.
Rallyes did corrode quite badly, but on the whole the bad ones dissolved some time ago and modern metal treatments have largely seen an end to the major problems.
They are not fast, but fly very nicely and in my view are very much a bargain in terms of what you can get for the money. Get a good one and I believe you will be very happy with it!!