I had no idea a 108 could have as an option the air ride system. Are they trouble? Not at all if your mechanic knows the system. On the West Coast, there might be 10 shops that know the system. Most mechanics cause more problems than they repair.
Is the spring system more economical? Absolutely, you get what you pay for.
Wheels, can you go 15?. Yes indeed. You can even find, at a price, exact matches to the 14 inch stock. 15 inch wheels give you numerous tire options. You can actually fit 16 inch wheels.
Freeway driving? Just buy a tire one to two inches taller, 29 inch tall tires will fit. But then your speedo is off by the percentage of that tire increase. GPS Speedos are available and advertised in Hot Rod Magazine.