I have a 97 S320 and I absolutely love it. When I need to drive in traffic I dont use it, but if I'm going on the interstate its the only way to fly. I heard the 95's still had some growing pains but that would not stop me from owning one. I put a little over 4k miles on mine since I bought it at 62k miles. I had a few things fixed ($650) that were wrong with it when I bought it and now she is perfect. Best money spent on a used car is money spent on a W140 in my humble opinion.
I have owned two in-line six W140s, an S280 and a S320 both of which were excellent cars. The only real weakness on the M104 engines is that the cylinder head gasket will leak and at some point will need replacing.
On my S280 the leaking started at about 62,000 miles and I replaced it at 99,000. The parts themselves are cheap, its the labour thats expensive. Find a good independent mechanic and you will save yourself a lot of money.
Other than that the car will be reliable if it has been serviced in line with the recommended schedule and on time. Avoid cars that have little or no history. These are the most complicated cars DB ever made but if you buy a good one there is simply nothing comparable. Oh and don`t worry about power. The S320 has 231bhp and is more than adequate.
I've had my 1995 S320 now for over...11 days! I am the fourth title holder, but the previous three were friends/family. It came with complete records, including the original sales contract and window sticker. This is what tells you to buy or walk: the records. Read them carefully. There's a certain comfort in knowing the car was well cared for, especially since I've heard these cars are not forgiving (when something goes wrong, fix it right now and fix it right). Was it maintained by the dealership or a specialist? Were major services done? I notice some cars are better cared-for early in life, but somewhat neglected later when they need TLC the most. Records of major repairs like wiring harness, A/C, head gasket are all plusses (you're less likely to deal with these common problems in the near future). In terms of style/ride, that's purely subjective. I think the S320 is a FANTASTIC cruiser, very comfortable, and quick enough. Take the time to read as much of these W140 topics as you can stand...it's quite the education!