Old cars are a money pit unless you are capable of doing the work yourself and enjoy doing the work yourself. Even then, parts alone can be costly. If you can’t do such work or hate doing such work, paying a mechanic is paying for his holiday, especially if they take advantage of your lack of knowledge.
As for cheap used cars, people sell them for a reason, usually cause they need work which will cost $$. So you are taking a punt buying an old out of factory warranty car unless you know enough to check things out yourself or pay for a mob to check it for you... but even that is no guarantee...
About the only second hand car one won’t have costly issues with is something coming off lease and with a few years of factory warranty left. Such cars are maintained as the factory required (but always check the log book and if possible check with the owners insurance company for accident repairs). Such cars are generally great as the owner simply wants to roll over to a new lease vehicle and not selling it because of issues... Problem is they are much more expensive than an old car... You can always look at cheaper lease cars rather than Merc’s or Bimmers. Consider the cheaper end of the market, smaller Toyota or Hyundai or Kia and the like. You may find something in your price range. Give a lease company a call and ask, you may or may not be surprised... A two second google found this mob,
streetfleet.
After all three year old car ($20k new) with 2 years of factory warranty left may only cost $12k and would only need yearly fixed price service, and every few years brake pads and tyres...