I paid a little over $4k for mine a year and a half ago. See http://www.carcentric.com/Ludwig.htm for the details of what was wrong and right with it.

By the time I get done, I'll have $9-10k into it, but that's with a new repaint (all the old paint removed), a newer 5.6 drivetrain, custom wheels and new tires, new sound system, stainless fender trim and sills, new seals and mounts, etc. - in other words, just the way I want it. I'd recommend my path only to those who really enjoy projects, however.

For the same price, $9-10k, you should be able to find one with no rust, no oil burning, and no interior problems (stay away from blue interiors, they seem to crack a lot more frequently than other colors). If you have a chance to buy one without a hardtop, talk the guy into dropping his price $1000-1500, then buy a used top for $300 the next time one shows up on in your city on eBay. Don't think the same way about the soft top, though, get a good one - the labor for a new one is more than the cost of the replacement. And don't buy any of them without a $50-75 inspection from a competent MB mechanic.

Another advantage to buying a 350SL is that sellers often don't know it has the 4.5 engine, so you can compare the one you've found to the some "bigger" 450SL elsewhere to leverage down the price. Even if they've heard the 4.5 is in both models, can they prove it?