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

Yes the car is older than 25 years old.

The flitch panel (my friends word not mine) is the metal inside the wheeell well that the lower hinge is attached to.  The lower hinge on the door is rusted through and makes closing the door difficult.  

Engine has very low miles.

I have some good pictures of the undercarriage and it really looks good with the exception of those two places.

£800 is a lot for a Mini that definately needs such work.

If the hinge panel is gone there will be other stuff to do when you start. I
bet the area around the air vents will be a bit lacey.

Try and get one that is cheap enough to so it makes doing the work
worthwhile or get one that doesn't need any work even if you have to pay a
bit more. If you do buy one that needs work get all the panels required and
ship them at the same time.

panel, and the left bottom sill?  These are the only places that the rust
has gone through.