Even if the engine is rebuilt and he's got some proof of it and there is no significant rust AND the paint isn't bad, then 5k would still be too high a price as a starting point. (Some may note the edit I've made to this first sentence!) The L-Jet may very well have issues and finding / fixing them yourself can be challenging and finding a technician who's able and competent can be the same, not to mention pretty expensive. A Weber set up will run you about a $1,000, without installation and tuning. Perhaps more, IDK.
It may not be as easy as you may expect to determine if there are any significant rust issues, but if the jack holes and round jack mounts underneath are solid, especially on the passenger's side, and the doors close nicely, then whatever rust issues it may have are likely not too serious. Check the gaps between the rear of the door and the body. Ask him to use the stock jack to lift the car on the passenger's side and see if that gap increases at all. If it does, the car has a problem in the "hell hole" and will require welding and parts which can be acquired from Restoration Design. That alone would also cost a significant sum to repair. Look for signs of any accidents, especially in the front trunk. No matter what, you need to expect to invest at least 5k to get it together and the interior looking good but that doesn't mean the car would be worth that much at that point and that's just the beginning! There are numerous other things the car will certainly need, each of which will cost, on the average around 100-200 bucks apiece. Brake lines, fuel lines, gas tank restoration, electrical issues, clutch cable, brakes, accelerator cable, ignition switch, Etc. etc. etc. Be prepared! A car like this that's not been cared for will need all these things and more before it's what anybody would really want it to be. The scope of the project may, thus, be much larger than you've expected it to be up until this point. Also, the 1975s aren't the most desirable models. The bumpers are heavy and, some say, ugly, and once you've back-dated the bumpers (assuming you choose to do that) and taken the L-Jet out for Webers, your car isn't original any more and has then lost any real "collectible" status. The 1973 2.0 appearance group and the LE cars are the most desirable and fetch the best prices.
You need to expect to put significant money and work into any project like this. If you don't have another 5k ready, then don't expect the project to be anywhere near completed until you do, and that's a very conservative estimate. There is low demand for something like this so with the amount of investment and risk involved only about $2,000-$3,000 can be considered a fair or at least. Somewhat good price, and that only if he's got receipts and specs on the engine build.
Was it resprayed the original color?
What wheels does it have on it?
Photos would be helpful!
M.i.M.