Firstly i would ask - is the car over 25 years old (the classic mini was made right up to 2001). The reason i ask is the car has to be 25 years old for you to import it to USA (just your import rules im afraid). As for the rust:
- what do you mean by the flitch panel?
-sills are ok to replace - never fit an oversill - always cut out the rust. Get it done properly as sills are very structural in a mini.
RS