Based on the fact that a lot of "gray market" tractors get a fresh repaint and some limited mechanical repairs I'd say in general a repaint will increase the value of your tractor, especially to people who are unfamiliar with them. I bought a tractor from a guy who purchased it in 2003 and he said he bought it "new" in 2003. I told him the particular tractor model hasn't been in production since about 1980. He was upset to say the least, and you can guess he paid accordingly for the mistake. I'm not suggesting you try to knock off an old tractor as new, but my point is that a fresh coat of paint makes that kind of difference to some people.
Also consider that to do a reasonably good paint job takes a lot of time, some equipment, and supply costs and this should be reimbursed, at least partially, to the person who owns the machine.
Have a paint shop quote you for cleaning and/or at least the repainting and I'll bet you'll be surprised how much it will cost. This should be reflected to some extent in the sale price.
As to the value of the machine as-is, that will be all over the place depending on who you are dealing with. Most people looking for these tractors don't want to pay much for them and a lot of times the owners don't want much either. Personally, I find them to be a good value for this reason. There is another recent thread that discusses the market value of a very similar machine. You may want to read that if you haven't already. Unfortunately, you'll find the individual used parts of the tractor added up will quickly exceed the going price of the entire machine. A lot of people not interested in having operating tractors will capitalize on this fact.