The cost of buying an old Mercedes should include 50 to 100 dollars for a complete inspection. If you do that you will get a car which has a total cost of ownership much lower than most any other car. There are plenty of "money pit" cars out there but if you are patient you can find a good one. Many used Mercedes cars were owned by rich folks that took them to the dealer every 3,000 miles and babied them. You just have to look hard and be willing to "pay up" so that you can find one.
I bought a 92 300D with 108,000 miles on it for my daughter for 12k in 2000. The prepurchase inspection done by a local independent Mercedes shop came up with a list of things which had to be done to the car by the seller and revealed that the car was well maintained and a good buy.
All of her friends got new Camrys and Accords (at a cost of about $20,000 each), but they liked her car better. In the first year, I bought a set of tires and changed all of the fluids and bought her a topsider and that cost about $500 total. Add in about $2,000 for depreciation and the total cost of ownership aside from fuel was around $2,500 for the first year. Insurance was cheaper and my licensing and property tax on the car was around $800 less than a new Camry would have been.
The Camrys and Accords her friends got did not cost anything to maintain because they were new and under warranty but they depreciated about $5,000 that first year. I felt that the $2,500 difference was a good hedge against some sort of catastrophic repair like an air conditioner failure. My daughter puts a couple of hundred dollars into an account each month (in lieu of a car payment) for that.
It has been 4 years and the car has gone 60,000 miles. Glow plugs have been replaced ($100) along with a timing chain ($200) and idler pulley and shock($150). Oil changes every 3000 miles (20 x $24) and total fluid replacement twice (2x $75). and a battery ($75) Brake pads are free on the Auto Zone lifetime plan. Thats about it.
I am getting ready to install a new water pump ($75) vacuum pump ($300) and alternator brushes ($45) as a preventive measure and the car will need tires($400) in the next year. After that she should be set The car is now worth about the same as a 4 year old Camry or Accord. An old Mercedes costs more to maintain but the depreciation is already out of it . It is a good transportation value if you do your own work using the tips that you get here on shop forum.
Owning one of these cars is like having a hobby that can pay off. If you don't have the time or interest, Get a depreciated 4 year old Camry or Accord that you have had inspected. They are very reliable cars but not as safe and classy as a Mercedes.
As a side note I have an 84 300SD with 176,000 miles that I have owned for 9 years and driven 100,000 miles. In that time I have had to do very little to it outside of regular preventive maintenance and about $6,000 worth of depreciation. I am in the process of spending around $2,500 on it to get it up to like new condition (seat repair, complete suspension rebuild, new chain, vacuum modulator for the tranny, new motor mounts and glow plugs). It now drives like a dream. It sounds crazy to spend $2,500 on a car that is only worth $3,000 in the market but I would have to spend that much in sales tax, property tax, and collision insurance not to mention depreciation for a car that rides as well. That is about 4 lease payments for the average guy driving around in a luxury car. So after about 4 months I am ahead.

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Tom Hughes
St. Louis
84 300SD
92 300D
86 300SDL