BMW automatic used car query

If the dealer couldn't be bothered in making the car presentable by at least cleaning it, for resale then it might give you an insight of what a 3 month engine and gearbox warranty is actually worth from them. 300k km on a 6 year old car is fairly hefty mileage and your really into unpredictable territory when it comes to reliability or how good that short warranty actually is when it comes to needing it.

Would you not be better with older bmw with less mileage?

Just personal experience but I always found newer cars with high mileage to be more reliable than older cars with low mileage. Resale can be difficult tho if you plan on doing high mileage yourself.

What garage is selling the car? And is it an in house warranty or 3rd party?

Realistically in the current used car market that is not going to get you anything decent in respect of your criteria. If you are ok with something a bit smaller then a Lexus IS300h petrol/hybrid is worth looking at.

I think either of the above would have a lower cost of ownership over a 300k km BMW or Merc.

OP you'd be wise to take the good advice you've gotten above to be honest. 300k is huge mileage on a 6yr old car. That car would have required oil changes twice a year with that mileage and I bet it hasn't, probably hadn't even had annual oil changes but only you can check that on the service history. Plenty of nicer cars about with that budget.