Make sure it has good filters & gets very, very frequent oil & filter changes.
Use a/c that have a few more seats than you need. Just don't put any bums on them eg a C172 but limit to 2 POB, or a 6 seat twin limited to 2 POB etc
You could consider turbcharged a/c. Cost more to run though. Another alternative is a turbo-normalised engine. You could anticipate non-turbo TBOs to be specified instead of the usually reduced TBOs for boosted engines.
There was a thread about this quite a while ago in another forum (Flying Instructors?). Do a search & see if it's still available.