A Mercedes is not a Corolla. That's the first thing you need to understand. They thrive on maintenance. These are technically complex cars and should not be serviced at your gas station. This car was designed to reward the driver with an outstanding driving experience, not a "Maytag" experience.
That said, they are also very reliable cars in terms of major mechanical failure. I have yet to meet an MB owner that's worn out the bottom end of an engine.
The M103 engine in the 2.6 can go for 400,000 kilometers easily. The transmission can easily last 300,000 kilometers. The body won't rust for decades, and remains tight and strong nearly forever.
To your specific points:
1. I don't have a ton of money to spend on car repairs. Would this year/make of Mercedes be a good investment for me? (These cars have such good retail and trade-in value -- even if I drove it for 10 years, I could still get 1/2 my money back!)
This is a nine year old car. ANY nine year old car is going to cost a reasonable amount to properly maintain over time. Mercedes' parts cost the same as most Japanese imports (much less in many cases) and are easy to do basic service on. Count on spending at least $1500 a year to properly maintain this car.
3. From doing online research, I've seen sporadic problems with head gasket failures and troubles with the a/c. They were other model years, not the '93. Is the '93 prone to any major mechanical problems?
The 93 M103 engine is pretty well sorted out, and very durable. The M103 has a lower incidence of head gasket failures than the DOHC M104. AC problems include pushbutton units, which have jobber-rebuilt units available at a low price. Same for cruise amplifiers. Final drives are long-lasting too.
4. What about service? The car has 70,700 miles on it now, and I'm not sure yet as to its mechanical history, although the dealor tells me it's in immaculate condition (but then again, don't they all?). Should I expect a service appointment soon? About how much should I be prepared to spend?
Any nine year old used car probably needs $2000 to bring back to snuff. I'd be VERY wary of a car without a service history. If you go for it, then you'll need to do a service right away to make sure it's in top form. Change every fluid and filter, including the transmission fluid/filter.
5. What about the timing belts and chains? I kept reading that these cars are very sensitive about these belts and chains, and I want to be sure that I don't mess something up if they need replacing. How would I know how when to replace these?
MB engines have no timing belts. They use chains. The M103 inline engine has a short chain that does not change direction, and is very long lived. A competent MB mechanic can check it for stretch. Timing chain life REALLY depends on oil changes and the treatment that an engine has received. On my 190E cars, I usually changed the chain and tensioner at about 200,000 kilometers for peace-of-mind. It's about $400 parts&labour at MB dealers.
Overall, how reliable is this car?
It's going to depend on the car you're looking at. A multi-owner non-serviced abuse case could be a nightmare of problems. Your best bet is to have the car carefully checked out. A well cared for 190E 2.6 can be an excellent daily driver, giving years of reliable service. Major components are nearly unburstable. For your maintenace requirements, the dealer can get you a service booklet.
But, again, I wish to give you a caveat. MB cars are the most relaible of the high-performance cars made. But, they are not rewarding to an uncaring owner. If you're looking for an applicance, spend your money on late model Civic or Corolla. If you want a car that can go 500,000 kilometers with the right care, and still blaze down a highway at 125mph all day without making you feel like you've been in the ring with Lennox Lewis, then buy a Mercedes.
1998 C230 "Black Betty"