We have a Tartan 30, 1977 vintage. Excellent boat. The diesel engine is an upgrade as these were all built with Atomic 4's. I recently removed the engine and am putting an electric engine in. I imagine the wiring changed quite a bit with the diesel upgrade.
A good coastal cruiser that performs well and can be raced but I wouldn't do extended offshore cruising on it. Pretty cramped for a liveaboard. Resale even with a diesel is 10 to 15K max so don't pay much. If this hull has been on the hard for several years examine the hull closely for blisters which will be very difficult to detect having dried out. If you fit it out yourself you will spend up to and more than market value before you're done but to finance a boat at full price (assuming you don't have the cash) would cost you double or triple in the end game so toss out the whole net net argument of cost. Paid cash for ours and all told have about 12K cash in it including sails, running rigging and other consummable items as well as the electric engine conversion.
There are a few minor issues with this boat - bow stem plate bolts had a tendency to corrode and fail so check that out. Chainplates, as most boats with thru the deck bulkhead mounted chainplates will leak and require frequent rebedding to stop the leaks (once a year for weekend sailing using 3m 4200 or 5200 but not a big deal). Engine is as easy to work on as it gets but when working on it, the rest of the cabin is consumed with tools, engine cover and etc - tight space for living aboard. Boat is watertight and dry as a bone assuming the deck gelcoat hasn't cracked anywhere (easily fixed).
The fact that the headliners were removed is indicative of leaks probably originating from cracks in the decks, the hatch area, chainplates, stanchions, etc. all of which would need rebedding or repair. Regardless I would walk the deck. Look for flex or knock on it with a mallet or something and listen for hollow sound. If so, its indicative of rotted core material from long term leaks and would need repair. Repair in such cases is easy unless broad areas of the deck are affected.
Winches and other deck hardware are no longer made or parts are very hard to find so replacement is more the norm. This includes the cockpit bilge pump although I think defender sells a repair/overhaul kit that will work at 1/2 the price of going new. Normally these pumps (Henderson's) will still work but the diaphragms will certainly be cracked or torn.
Check the keel bolts as these are normally stainless and as far as I know don't have or didn't have any protective coating on them. If there is rust it would be wise to investigate further especially the keel/hull joint. If you see rust stains coming out of that joint, it will be a bit expensive to have the boat and keel separated to replace/reinforce those bolts. I believe, but don't know for sure, those bolts are molded into the keel making replacement nearly impossible leaving you with drilling holes and installing additional bolts as a possible solution.
Check the keel stepped mast at the keel for corrosion at the base. If the metal is heavily corroded, clean it up and be sure the mast is not damaged, pitted or otherwise structurally unstable.
Look for play in the prop shaft to be sure the cutlass bearing doesn't need replacement and check the shaft at the stuffing box, thru hull and the grease fitting to insure it has no grooves or other wear from years of use or abuse.
These are all things that will add significant cost to your restoration. If the diesel is well cared for this is a plus as people are spending 10K or more to replace the A4s with diesel (including installation costs) but don't count on it adding anything to the resale value.
Hull repair is relatively easy but I would strip the bottom paint all the way down. I put an epoxy barrier coat on, applied 3 coats of ablative and redid the boot top stripe. Stripping the paint down to gel coat also exposes the tiny pits indicative of dried out blisters
When you drop the boat in the water it is designed to sit lower at the bow waterline which is offset by the added weight of crew in the cockpit while sailing. S&S designed it that way so if you notice it, its not a concern.
That's about all I can think of. Good luck with it if you get it.