1. I don't have ASC so I don't look much for it on this site. But then again, I don't see many posts about it. I'd say not common. What is fairly common is to see people with BOTH the ACS and ABS lights on. That's usually a wheel speed sensor broken as they both use the wheel speed sensors for proper operation.
2. The engine will last for a very long time. It's rare that
any modern engine "wears out". Like bad rings, bearings, cylinder walls, etc... If you are willing to replace things like the alternator, sensors and little misc part, you could probably run it past 500,000 miles if you really wanted to. I'm at 189,000 and it doesn't use any oil over my 7,500 mile oil change intervals. It still easily does 35+ MPG on a freeway trip too.
Your #1 job for durability with these engines is to prevent it from overheating by following the advice in this thread.
http://318ti.org/forum/showthread.php?t=16068 Replace these two parts unless you know for sure they have been replaced recently. It's probably the only significant issue with these engines. Thankfully the parts are about $10 each. There is a bit of labor involved in replacing them though.
3. Mine is slightly lifting at the corners too. I wouldn't worry about it, other than for looks.
4. If you are going to buy a full set of shocks, springs or both, you need to buy a 318ti specific set. The suspension is totally different in the rear vs. the standard E36 cars.
The gear ratio for the manual transmission cars is 3.45. The easiest (mechanically) option for LSD is to find a stock 318ti diff with LSD. Simple swap then.... Those are hard to find though, so there are other options from earlier BMWs (like the E30) for LSD swaps. There's a forum on this site dedicated just to differentials.
http://318ti.org/forum/forumdisplay.php?f=32
Linear Mode
