I think its more of what is left thats not rusted?
What you can see is just the surface rust, rusts are like cancer you wont know its there until that first symtom and its already too late.
When I bought my car I checked thorroughly for rust and still when I stripped her down I found more rust. I wouldnt worry too much with cosmetic rust since its going to be a stripped down race car, just check for structural rust as suggested by other members.