Any repair on the newer model you cannot handle might financially kill you even if you can stagger along on the payments. Plus you would still owe your parents for part of the 240d.
There is never a better time than now to sit down and think of a way to repair what scares you but not cost much if any money. Once a car has had or has extensive rust one has to use judgement on how much to spend on it. Thats especially true with certain types of rust and if it is wide spread.
Very difficult to make a car like that truly whole again and usually not worth the effort. The equation of an old car no money and a mechanic does not fly well to start with.
More self reliance also helps you through life in my opinion or at least you learn what is going on around you. The more you know the more you will find it almost unbelievable in my experience or at least a lot of it. Most important is you can usually also make money with that knowledge.