3 litre and red seats sounds like a good plan. It looks like you have had mostly newish so probably low mileage cars but if the history and condition were good I'd be flexible on mileage. I'm on my 2nd 80K+ Z4 and my third 100K+ petrol 3 Series and the engines have all performed well. Any issues I've had were peripheral things like cooling systems, leaking shocks, worn out top mounts, etc. that are just as likely to afflict any
15 year old car.
I can relate to that as my first car was a 1967 MK2 Cortina on its 2nd trip round the clock that was a clapped out rust-bucket!
But my eyes were opened when I bought a 1992 Cavalier SRi ex-company car from my employer in 1995 at a price I couldn't ignore, and took it for its first MOT with 97K on the clock! But it had full main dealer service history and was a great car. After that I had a Mercedes 190e then a C280 both with 6 figure mileages and they were fine too.