Sounds like you are having a rough time. I can understand your frustration at having so many problems with a car as young and well cared for as yours. Since my 420SEL is a w126 and not a w140 like SEL I can't give too much insight into your specific problems. All I can say is that you are driving probably the most complex car that Mercedes has ever built. Apparently, only the 600 of the 60s and 70s is more costly to maintain. The more features a car has the more areas there are to develop problems. Most of the problems that I have had with my car are what I call superficial. That is they don't really affect the way the car drives or handles, or how safe it is. Most have been related to the sunroof, door locks, antenna, etc. The reason those of us with older cars love them so much is that rarely is there a problem that affects the real pleasure we get from these cars, that is the driving of them. As Mercedes owners, generally we are perfectionists. We like to keep our cars in better shape than we recieved them.
As for what to do, the first step is to look. Find out what you can get for your car, as is. Then look for other cars you are interested in. Instead of looking for an older car, which many of us on this board enjoy, try looking for a newer car with a warranty (star mark). Sounds like you could use a break from paying for repairs for a while. Either that or live with the minor problems in your car, and fix them as you can. There's no law that says you have to fix your power seat right now (driver's side might be necessary, but passenger side is no big deal). It all up to you.