300s do rot on the sills (and various other places), although my 240 is just as bad as my 300s! Admittedly, my 240 is unusually rusty. 7/900 series cars are the ones which barely seem to rot at all.
How tidy is it in general? How mechanically sound? Rust is the big issue with these cars nowadays. You'll be paying over £500 to get a top notch example, so if the sills are all that's wrong with it...
If you do get it done, make sure they're not going to patch it up to bodge it through the test....and make sure it's painted properly. Otherwise, the rot will be back within a year or two.
cheers
James