You can take it to BMW for a health check.
They will inspect the car thoroughly as they want the business!
Search for BMW health check in google and have them do it.
I think it costs around £40-45
Also check MOT history and look for advisories and ask if they have been dealt with.
If they find anything you could use as a negotiation point.
Keep the other £2000 spare for service / maintenance & repairs. Expect some issues, but that is part of car ownership.