check around the windscreens - one of the biggest signs of structural rust (and very hard to eliminate totally).. it will probably mean there is rust down in the firewall (in my experience).
pull up all the carpets to check the floor pan.
check along under the front of the car - the sub-frame can rust out in the front and make the front end not very safe.
as always - check in front of and behind the wheels (especially behind the front wheel and in front of the back wheel)..
that's all I can think of for now.. but in many years I am yet to find a cheap ta22 with no rust.. unfortunately they are very prone to rust.
I've always steered clear of coastal locations when looking for old cars, for obvious reasons..
good luck! - take a magnet for the old magnet test ..