I don't know enough about the 570S or 650S to comment on them. I imagine the reliability of the newer models to be improved, but I can't say that with any certainty.
I was compelled to look up the current value of the dollar vs pound since I've seen some nice MR-Ss over there being advertised for peanuts in great shape, plus $100k doesn't seem like a lot for a 650S. Today it's £0.83 / $1.00. Not bad. I think our used car market in the US is just bonkers right now and it may be more normal in the UK.
Shame about the rust on the FD! It wasn't showing up as bubbling paint? I hope it's not too dear a fix for you.