I just recommend buying one from a main dealer as it's got warranty. Even the smallest repair is usually costs a chunk of change.
Alternatively you could ensure it has full service history, enough to qualify for a Mini extended warranty, we did this for my daughter, she saved £3k over main dealer prices, got a much cleaner car and it was £350 for a years extended warranty..
However, buying privately has less protection, so if not confident, a pre-purchase inspection might help (but again, they can be hit/miss)..
Essentially buying from a main dealer is always the least hassle route and if unsure gives the best level of protection/warranty.. (although that's no guarantee!)..
Private sellers can be weird, we bought a 4 year old countryman for my wife back in 2015, the owner had the last service done at some fred in a shed garage, so we negotiated a bit more off based on not having the full service history it was advertised with since there was no detailed invoice for the service.. On buying the car, it turns out it was still on it's 5 year TLC package and so we got it serviced by mini for free!