The Tartan 34 is a fine boat. It is absolutely among the best looking boats out there. I think it was drawn by S&S?? Regardless, it is a sweet looking boat and will turn heads in any harbor. Bow, sheer, stern, brightwork....not much to improve upon.
The centerboard style keel is well designed. It is robust and will give you no problems. It is great to be able to crank up the keel after a grounding, or enter shallow harbors.
If you are performance-oriented, you can add a little strip to make the joint between the board and hull pretty tight. You will also enjoy really nice pointing ability with the board down as you go upwind.
I keep my boat (1975 C&C with original Atomic 4) about a half-hour from Sandusky. The A-4 is a solid running engine, simple to work on, and will run decent even on really bad gas. Unless you have SERIOUS cruising plans, don't bother changing it out. To swap it for a diesel will cost you about 9k for the engine, plus new fuel tanks, fuel lines and engine instrumentation.
Just run the blower for 5 minutes. No problem. There are tons of cheap A4 parts available, and the undisputed guru is Don Moyer Marine. Virtually every part can also be cross-referenced to NAPA to avoid the high dollar marine mechanics.
Remember these boats were built during the 70's. Before the petroleum issues that caused boat builders to go cheap on the resin/glass ratio. There should be no hull blister problems here. Tartan was, (and is) a Class-A builder. They didn't skimp on parts or labor.
If you want, you can use the diesel issue to your advantage, and tell the broker you really want a diesel, but might settle for the 'dangerous' gas if the price is right. Ditto for the time on market. However, this is the midwest, and Sandusky is smack in the middle of the auto industry. We have had a rough couple of years out here. Since she's in Sandusky, its a fresh water boat and should have no big corrosion or rigging issues.
Good luck. I would get a classic boat like that in a heartbeat. You'll also like the cockpit seat hatch access directly to the cooler/refridge without having to go below for that cold refreshment.