The fuel system is likely borked; My car sat for a few years (climate controlled storage, but still sat) before I bought it. The fuel pump and fuel sender unit were shot, fuel filter gummed up, and the fuel tank had to be cleaned out as the tank was coated in varnish. I've run 2 bottles of the more concentrated Techron though and haven't had any issues since.
Other issues a 986 can suffer from sitting include:
- Rodents; I found a nest under the intake crossover tubes.
- gummy window seals; mine were streaking on the windows for some time. Seems to have lessened but when I take off the door cards to solve a minor rattle I will need to inspect those seals and the window regulator
- The blend door foam is likely shot, you'll see it spray everywhere the first time you crank the AC/heat
- Brake fluid needs to be changed ASAP; any water in that system may have already started corroding fittings
- Coolant too should be flushed ASAP
- Oil has pooled away from components, and should be changed before running
- The frunk release may have seized, you'll have to manually release it to get to the battery... which is most likely dead as well
- Tires are dry rotted and unsafe, I drove 12 miles on the old tires (which were only 7 years old) and it was mildly terrifying. No grip, crashy, wandered everywhere. Should be changed while car is at a shop getting reconditioned
There's much more that can rotting away but internal components *SHOULD* be fine - once the oil is changed, fuel system cleaned out, and coolant flushed. In fact the water pump should be changed before starting, as should the serpentine belt. The Throttle should get cleaned, as should the MAF. The AOS rubber bits are probably rotted so get that changed too. If there's any oil leaks it's likely either the RMS or possibly a head gasket (but unlikely.) Plugs are likely fine, and whatever condition the PO left them in. The coil packs however are likely dry rotting and beginning to show microcracks. Mine are. Not a big deal as I don't drive it in the wet, but one good rainy drive will likely lead to misfires so that's gotta get done. The IMS should be fine however if it is undamaged. But you'll prob want to pull the gearbox to have the clutch examined. I am experiencing a mild clunk in the front suspension over harsh roads, indicating those bushings in the track arms and lower control arms are past their best. Same goes for the rear, any rattles you hear (again, mine has over harsh pavement) is either the track arm, toe arm, or lower control arm. Not catastrophic but likely a symptom of sitting and drying out. Then there's the end links and tie rods...