I'm always wary of barn finds because since the car hasn't been driven in decades, you have no idea of knowing what is and isn't functioning.
Generally you should replace all your fuel lines in the engine bay as well as radiator hoses. Inspect the pigtails of your injectors, AFM, TPS and look for corrosion...you'll probably just need to clean them.
I'd personally swap the thermostat.Inspect the battery cables and replace the battery.I'd do a cursory rust inspection on frame rails, battery tray, rocker panels and around the A and B pillars (though they don't really rust there till it gets bad)brake pads, new brake fluid.
I'd get a new starter and alternator and volt regulator and keep them ready to go. Those parts may last another 10-15 years or they may go as soon as you start driving her.
Get and FSM and verify that the coil is good, alternator is charging.New plugs/cap/rotor and I'd probably swap the fuel filter... then gas her up and see if she turns over.More than likely she'll turn over with a fresh battery, plugs and some gas...but then you're just waiting for things to fail...which won't happen quickly...but will eventually happen. If you have the time and money swap as much stuff as you can before you really start driving her.
I would definitely do all the things I've listed above before you start daily driving it. But to be completely honest L series run forever. As long as they don't overheat or run out of oil the motor will give you little if any problems.If you are being really thorough...get under the car and looks at the brake lines to make sure they look good...see if you see signs of any leaks...but that would be obvious in a car that's been sitting.test your shocks/springs...you're going to want to replace them sooner rather than later.
Value isn't something I would think about. Contrary to popular belief most S30's don't sell for 20-30K and these cars are a horrible investment. If you're looking to capitalize on the JDM fever and eventually turn a profit on her, you're most likely going to be disappointed. If you want to get a feel for what other S30's are selling for get on EBay and search for completed listings and look at the cars that sold and the cars that didn't...then contrast that with your local craigslist...that will tell you what the market is really pricing the cars at...but I'm a purist...so I don't believe in selling them to make a profit.
Have fun..and remember an S30 is and S30 is an S30...there's very little difference between all the classic Z's
1977 280Z, 3.1L Stroker , Ported N-42, Lonewolf performance Intake, 3 into 2 headers, Megasquirt I-V.3, Kumho Ecsta XS's (supa sticky),
This 280 ain't no old lady