I used to have one, the firewall and frames (rear crossmember especially) are the major rust areas. Door bottoms also rust. Then there is the issue of where the aluminum from the rear tub, door skins and front "wing" panels come into contact with steel.
Ones in good shape are still $18K and up but beaters have come down in price to around $10k. The NAS (North American Spec) trucks are the ones that get the most money. Due to the 25 yr import rule people can get ROW (rest of world) trucks for less, but again beware for rust. Most NAS trucks are now over 100k in miles. Check D-90 source
http://www.d-90.com/. Best place and people for things related to the rover 90s and 110s. For additional info regarding restored rovers and pictures of what to look for go to ECR.com