I'm considering saving a 1970 220 D/8 (115.110) from the unkind fate of sitting to rot under my father's barn. Its a neat old car, not too much rust, etc. The engine was rebult in 2001, after which my father drove it for a year or so. Its been sitting ever since.

I rescued it about 2 months ago and having been driving it ever since. I've replaced the leaking fuel line, replaced the belts, and had the oil changed.

Shifting from 2nd to 3rd has become an ordeal. It is physically hard to shift the lever (floor mounted). Its almost like it gets stuck in no-man's-land. It doesn't happen every time, or even every day. The clutch doesn't seem to slip, and its easy to get into 1st gear. I took the car in to a guy who works on old mercedes. He tells me I need a new clutch, and that it will run $1300! Do the symptoms I've described sound like I need a new clutch, and if so, is $1300 the going price?

I've been reading through some old posts on the board re: 220D's, and wonder weather I should change the trans fluid, the coolant, fuel & air filters, etc. I've never worked on a diesel or a Mercedes, but I'm game for learning how to. Are these things that (1) should be done and (2) can be done by me?

I've heard that Mercedes present a 'challenge' when changing the oil on your own, is this one of them?

Any other preventative maintenance that I should be performing?

Many thanks for the help!
(NB: I made a similar post on the Vintage section of the board but have expanded the post here in hopes of getting more advice, etc.)