If you are getting that W123 (300D turbo) I suggest you look into this website so you know what to look for:
http://mercedes-w123.net/buyers_guide.htm
http://mbdieseldiy.tripod.com/dog.htm
And if you want to join our forum,
http://www.mbnz.org/forums/forums/forum-view.asp?fid=10
How many miles does the 300D have? Check for leaks, rust and if the engine emits blue smoke when warmed up. If it does, walk away. Leaks and rust are a problem with these cars if neglected, but if this one is sealed and intact then you found a gem!
Anyway read the links I posted. And browse the forum site for added info. I have the same username there if you need to ask anything. Goodluck!
EDIT: The W123 is such a good DIY car you won't need much money to maintain it yourself. Oil changes, valve adjustment, diesel purge, etc... as long as you are comfortable under the hood you'll be fine.