On a 30 year old boat you have to make a lot of questions, for example: When was the rigging changed? (some say that should be changed every 10 years, some say more, but certainly a 30 year's old rigging is not fit anymore). The mast is the original? If so you have to look very carefully. Aluminum tend to turn brittle with time and to corrode on the links. The boat engine?....the freezer engine?...the pumps?, all rubber parts... last time the boat had an epoxy coat barrier...has the boat osmosis?, or absorbed water on the hull?
An old boat can be as new but that means that only the hull and the interior are the same. Almost all systems and other parts should have been replaced. Many that have old boats (or new for that matter) sail only occasionally and use the boat very little and just don't do an adequate maintenance other than visual.
Bottom point you need to know how much money you are going to need to put both boats to specs and probably you are going to need a lot more to do that in a 30 year old boat than in a 10 year old boat (that's why old boats cost less) but each case is a case
I think I am going to go with the newer boat, but if that offer falls through here are the results on a lot of the questions you addressed
The Tartan 31 is in very good condition as far as the hull, interior, cosmetically, and I've been told there have never been any blisters. The negative part is that it has original sails and mast. The standing rigging was replaced 12 years ago and the running rigging is very stiff. The bottom was painted 18 months ago.
I know I will have to replace the sails. What will that cost?
Do I have to replace the mast? How much is that?
Do I have to replace the standing rigging that was replaced 12 years ago. If so, How much is that?
How much is it to replace the running rigging?
How much is the bottom painting?