Hi Helmet I did my due diligence before buying a series 2 S280. I researched what to look for and what to avoid. Consensus is don't even think about series 1.
Here's a little note written by someone from the NSW MB Club - it was written some years ago so some of the details are a little redundant - but you get the idea.
I've got 2 S500's, sold the 3rd. Like Peter said, it is the best of the 140s. Do NOT go into ownership of this car thinking that since you've got the best 140 you're going to be saved. This car will cost you a small fortune to keep going. Some can afford this. If not you'll have to do all the work on it yourself.
All Mercs of the era had evap problems. The 140 is the most intensive to repair. My first one took me a month. Lots of people brag they did their first one in 2 days, they're full of it. Having done 3, I could maybe do another in 3-4 days non-stop. If it hasn't been done to your dream 140, it'll have to be. Don't fool yourself. Some of the older units are now going on their 2cd evap.
I can do one will all parts for about $700 if I don't bust up the ignition and steering lockout. You can have the dealer do it, they'll get you for about $6k if you're lucky. The evap, heat exchanger, drier, expansion valve, hoses, seals will easily be $2k in just retail parts. You can certainly buy parts online, but you can't take them to the dealer. You can also find a knowledgeable indy, that'll save you a fortune.
Some other problem areas is the suspension, hydraulic is standard on the 500. I'm sure you've heard the strut/accumulator woes. Struts are maybe $600 each, accums aren't bad - maybe $100 ea.
Heaven help you if the roof acts up.
Figure the window regulators will break at some point. Being built like a tank, it just takes the usual effort to take the doors apart and re-drill them.
Instrumentation, electronics usually won't give you much trouble. The controllers (like fuel, ignition, ABS, theft etc) will. Sometimes they last a while, sometimes they don't. This is old solder the component type construction and there's a LOT of good info around to help you diagnose your own boards. If not, you can try to find rebuilts. Finally you can usually get one from the dealer. Ignition controller #1 on my car is $1400.
The stereo is just a marvel, Bose made it for Merc. The amp is in the trunk. Dealer will get you for a lot of $$ for it. I've seen them on Ebay out of stolen cars etc for less than $200.
If your motor has been neglected, about all you'll have to do it have it cleaned or the usual timing chains etc replaced. Dumb stuff like the oil recirc tubes between the valves pop and cause the motor to tick. Loudly. Not a real problem to fix, again just time. It's the finest motor ever made by MB. Internal failures are rare. If you do need a new one, a reman installed will be in the $20k range. I've done 2 complete rebuilds of the 119 (long blocks), my total cost about $2k and lots of time. Tranny in the $5k range.
Headlamp assemblies for Xenons are $2k ea from the dealer. They are rare from 3rd parties. Also expect the usual issues from the blower motor, the occasional leak, clogged drains, O2 stuff, fuel sensors, OVP relays, all glass is very expensive. The steering/hydraulic pump can give you trouble, as can the water pump and alternator. But no more than any other car really.
And finally almost all dealers get these cars from auction or repo. One 99 Grand I remember changed hands 5 times between lawyers and other dealers before it was finally wholesaled. 99% of the dealers have NO idea what a 140 is. Or what to look for. They usually have canned feature descriptions that you can easily spot. Resale on 140s is awful. I knowledgeable buyer would never pay more than $10k for a 100k mile S500, and that's if all the stuff was in order. Sometimes Ebay dealers nail someone for $15k+, then fight after the sale over condition of the car. Buyer rarely wins. Remember Ebay protects their dealers at all cost - it's their lifeline.
A lot of 140's are being sold via the 30% interest and 50% down method, and the cars are often ghetto'd out. This is usually 20" wheels which will ruin a 140s suspension in a matter of months. To me this is a sad time as a lot of 140s are being "stolen", cut up for parts, or just hauled to the junkyard. There are VERY few S600s left, and the ones that are still running are owned by very knowledgeable and dedicated people. The motor was that car's Achilles heel, and there weren't many that made it past 100k miles. It cost about $30k to replace that motor, and is so despised by MB shop techs they usually argue over who's gonna work on it. The 137 model fixed most of that but was a lesser 3 valve design with some proprietary rights issues.
The length of this little note was intentional, I've seen more than a few inquiries of late RE S500s. Each time I want to say this [herein] to the excited buyer who'd found another Ebay car. In summary, a well tuned S500 in a 140 model is by far the best ride in the world. Not just of MB, but of any car. You'll never drive such a worldly machine. Just be ready for what it can throw back at you. Show the seller you know what to look for. They will ALWAYS tell you the same thing – “If you don’t buy it I know someone who will”. Stand your ground
And then there's this quote:
Unless you are competent working on these things yourself and have contacts in the industry for parts buyer beware..
The W140 will destroy your wallet, mind, spirit in Mercedes product and may even take your soul...
StarGarage.
But then :
If you can not afford to drive, maintain, repair, love & cherish your Mercedes........buy an " appliance
Buying a W140 for $10K and then spending $20K on it is better than buying a new E class for $100K and then losing $50K driving it out of the showroom door.