Heh - funny you should ask! I have two 300D's with the same motor and both need head gaskets (possibly heads.) Here's the bad news. You can only buy a brand-new head from Mercedes. They are $1300-$1800, depending on who you buy it from. That is a BARE head with guides only - no valves, vale lifters, camshaft, etc. New valve lifters are ~$20-25 each, new valves are ~$250+ for a set of 12. You can re-use the cam if it looks OK. You may be able to re-use the other old parts, but don't forget labor to swap them and have a machine shop check them out, etc. The lifters especially are probably worn after 200kmi. The head gasket kit, valve stem seals, etc are probably another $100-150 in parts. Don't forget that you'll need special tools to remove the prechambers etc. The head bolts are torque-to-yield, if stretched they need to be replaced also. While you're in there you should consider changing the glow plugs (IF they are old), they're a real bear to access with the engine intact. Ditto for the I/P vac shutoff - much easier with it all apart.
Overall, not a cheap project. I'm starting this myself (on both cars) and I expect it to be between $500-$2500 PER CAR, depending on what exactly ends up being wrong. I bought a bunch of the tools needed, instead of paying a shop to do it (was quoted a minimum ~12 hours labor for the gasket ONLY, much longer if the head is bad). DO NOT attempt this job without the factory OM603 engine manual!
BTW, ~$1500 is a decent deal if the car is in good shape otherwise, and is running. If it's not running I'd be very careful... you need to drive it and make sure the tranny, brakes, suspension, etc are all decent.
"There's nothing more expensive than a cheap Mercedes."
Good luck,