Welcome to Ub.c
Which e46 are you looking at buying? I would't shy away from a high mileage car if it has been maintained by an enthusiast. A dealer maintained car means squat to me. If the car has been maintained and their are receipts to prove it, the 6 cylinder cars are tanks.
I can't speak about specific trouble spots, but you might want to ask Hoppy. He had an e46 with a lot of issues. Good luck with your search.