Interesting why TB9/10 parts were hard to get. That is not current or recent experience. However there are some dumb maintenance shops who might order say an air filter from Socata when actually it is a £10 Brackett item from any old aviation parts shop and is used all over the GA scene. Socata don't help, by allocating their Z00... part numbers to everything in the IPC, so a bit of a brain is needed, not to mention a desire to not rip off the punter.
The TB9/10 is not such a good choice. They are basically much more comfy versions of the PA28 140/160, but few are in a good condition now because their sales were poor after the early 1980s (due to overpricing). Anyway the OP's £130k budget means he doesn't have to look there.
A quick google on the RV4 fuel flow suggests that your friend is getting about 10% over book figures, perhaps due to the prop.
But the RV4 is a tiny cockpit - 2-up tandem. Again, one needs to be sure that this meets the requirements. It's not suitable for holiday trips, carrying the usual stuff as well as a life raft, oxygen, etc. And
crucially few women will want to go anywhere in that way
