Offering $1,500 for a vehicle that is listed at $4,500 might seem a little agressive but hey, who knows how long it's been for sale or how badly the seller wants to sell? Seller might want a little more and if you like the overall condition of the vehicle, maybe it's worth a little more to you...
I recently bought a 1987 420SEL with 61k original miles; seller was asking $7,000 and sold it for $4,700. The interior isn't perfect and I'm still sorting out a few problems, but I look at the fact that it's 21 years old and vehicles tend to break and have problems, even if it doesn't have 100k miles on it. You can find dealers on Craig's List and eBay that sell Mercedes all day long; their vehicles are probably in far better condition, but of course, you'll pay far more for it, in part because your paying profit to the dealer.
With older Mercedes, BMW's and such, it's like the old saying: you can pay me now, or pay me later. One way or the other, it's going to cost you. Buy something really nice and pay top dollar today, or buy something older, and less expensive, and plan on spending more dollars on a regular basis.
I just came back this last weekend from a trip to Monterey; I was surprised just how many W126's you see in that area. Driving down and back on Hiway 1, I'd say at this point that spending $4,700 for this SEL was worth every penny. Great road vehicle and amazing how many people look it over at stop lights... classic body-style, I guess.