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The 3.3 and the 3.5 motors are basically the same, both are anvil tough and good for 250,000 miles if properly maintained. Indeed the only issue is the head really, it needs going through at the interval, or maybe sooner depending on how it was raised. The timing set and oil pump need to be replaced then too. The bottom end is nearly indestructible. I have seen engines with 500,000 miles that still show the crosshatching on the cylinder walls and turn in stock compression numbers with a refreshed head.

The thing you want to watch for on the Big 6 M30 motor is get them after about 82 or 83 when they switched to the Motronic engine control system and got away from thermal reactors and their heat. Also even on later models, if the owner has used standard Antifreeze then it is possible the cooling passages in the head have been clogged. The head is aluminum and reacts with stand coolant forming a soft fluff that clogs things and overheats the head. Bad news, but fixable. If the car is running good, just flush, fill with real BMW anti freeze or its equal and hope. Also there are banjo bolts that hold the oiler bar above the cam that can back out and starve no 1 cyl cam lobs causing them to wear. Again fixable and preventable.

If you get to the point of leaping, mail me, I can give you the hints you need for years of care free driving in a mid 80s BMW. They are worth investing in.