Don't worry about the "S4" badge - they were just stickers, and not all cars had them or have them anymore (were these dealer-installed?). Carfax is not perfect but it is a well known fact that there were only S4s sold in 1987. FWIW, Porsche does have an option code for "no model designation" or something along those lines, but I've seen enough 928s without the sticker and without that option code to confidentally assume it applies to different model badges - likely the "hard" Carrera badges on 911s and other models.
You are wise to worry about starting that car after so much time! The biggest issue is that the normally-lubricated engine components are now bone dry, and can be damaged significantly on start-up before they have been re-lubed. The second-biggest issue is the quality of the now-old fuel and engine oil.
Before rousing a dormant engine from its slumber, get a couple of quarts of Marvel Mystery Oil. Pull each spark plug and squirt a couple of fluid ounces of the stuff into each cylinder with a syringe, squeeze bulb, or a turkey baster (not the one from the kitchen). I've been advised to pour Marvel Mystery Oil over the valve train (by removing the valve covers) as well before starting a car that sat 15 years since all the parts would be bone dry - and I would advise you to do the same.... better proactive and safe than lazy and sorry. Turn the engine crank a couple-few of revolutions by hand (that is, by socket and rachet or breaker bar), plug the spark plug holes with balled-up rubber gloves or some non-shedding material to prevent ANYTHING from getting into the cylinders, and let sit overnight at least - a day or two even - periodically turning the engine a couple of times. Remove whatever you plugged the holes with, turn the engine by hand a couple more times, then replace the spark plugs (new plugs may be a good idea - I like Irridium plugs as my engine definitely runs stronger with them).
The fuel in the tank and in the fuel line is suspect at this point, and should ideally be drained and fresh fuel used to start the car.
Replace the engine oil, but the filter can wait until later.
Before starting, remove the fuel pump fuse and/or fuel pump relay (don't recall which there is) and crank the engine with the starter and fresh battery for a while (a minute or two) to circulate the oil throughout the engine. Next, jumper the fuel pump relay socket to force it to run and verify that there are no fuel leaks anywhere. I would strongly suggest bleeding the fuel lines and rails at this point to get all the old fuel out of the system.
Get a fire extinguisher ready to go, replace the fule pump relay, roll the car outside (there will be a lot of white smoke coming out of the tailpipe! You should even consider removing the catalytic converters to avoid oiling them up), then - finally - try to start it.
But don't get your hopes up - 928s do not like to sit in a garage undriven while all the other kids are out playing. They get angry, resentful, and they hold grudges. They express their displeasure by petulantly breaking themselves, like a dog left home alone too long that pees on the best rug in the house. All manner of other problems may have developed during the down-time, and masticulating mice know how to take their toll as well (ask me know I know...), so the car may not start at this point, or a lot of other problems will present themselves when it does.
Anyway, if it starts, immediately check the dash for any warning lights and check the engine for any oil leaks. Check to see if the A/C is operating.
Flexible brake and clutch lines tend to swell shut on the inside over time while appearing perfectly normal from the outside and the fluid will not flow freely, causing the brakes to hold after the pedal is released, or the clutch may be slow to close after releasing the pedal. Pump the brakes a few times to verify that the pedal is firm, press the clutch and have someone push the car to see if it rolls freely, then see if you can put it in gear and move it a few feet. Once the smoke clears from the exhaust, shut it down.
That is my take on what to do. The Rennlist 928 forum is the place to figure out how to do it. A decent mechanic should be able to do most of it without a lot of help. I would strongly consider getting more input on what to do from the Rennlist 928 technical forum. They have way more collective knowledge and experience than I do.
Lastly, consider having everyone's liability spelled out in writing, in advance - something could go horribly awry (like a fuel leak and resulting engine fire) and that's not the time to decide whose problem it is.
GOOD LUCK!!!
