As for things to look for, I would run a compression check and make sure you have similar numbers on each cylender and are around 100+ . I would crawl all over it look for the any hint of bondo, or undercarrage rust. I would drive the car (if possible) and see if the steering, suspension, ect, is okay. If the car just has 16k on it, odds are its in pretty good shape.
If you have any questions on the gt, please ask. Their are a lot of knowledgeable and friendly people here on the forums. Good luck to you.
I have seen some really nice GT's go around $5500.00- $6500.00 on e-bay that have been reconditioned or near mint original. Some sell even more when they have been updated throughout. Some that need work to be nice are half that. I have seen some real crappy ones that are driveable sell for $1,000.00 - $2,000.00. I wasted some time looking at a GT the owner said was in pretty good shape for its age, turned out the front nose from mid fender forward was spliced and welded together, the windsheild didn't meet the car in one corner and it had a lot of bondo in the door jamb near the roof. Also when I sat in the car my head hit the roof which doesn't occur in the other GTs I've sat in. He wanted $3000.00 !! Most people sell for all they can get out of it. A lot depends on how bad someone wants to get rid of the car or how bad they want the money. 16,000 miles sounds too good to be true. Opel GT odometers return to zero after 100,000 so keep that in mind, although 116,000 or 216,000 miles on an Opel doesn't mean it's junk by any means. Let someone have a look that knows what they are looking at and where to look! Good luck.
When I got my GT the odo said 63xxx. We figured 163xxx since there was low cyl compression. When we got the engine apart there was virtually no ridge and you could still see a little hint of the original cross-hatch in the cyl walls! It must have been 63k original miles and just sat so long or been overheated to take the temper out of the rings and cause low compression.
Nice paint, rims, new interior, spolier, it definitly LOOKS good. Again, check out the mechanical side, if the compression tests out I would be willing to put down 5,000 for that car (now if only i had 5,000). Where are you located again? Here in the midwest cars tend to go for a little less. But either way, I think you might have found a pretty nice gt. Keep us updated.
The car looks great! But you will need to pull the carpet back and inspect the condition of the floorboards. The brown stain coming out from under the car looks like someone cleaned the corrosion off the battery. For grins, I'd ask to see any receipts or documentation of parts purchased and repairs made.
Remember.... the seller will (and should) ask for a price higher than they hope to get ... You the buyer should start with a bid lower than you think the car will sell for.... I know it sounds silly for me to say this, but some people will get intimidated by the seller and will make their first offer too high spoiling any chance for a great deal!
The closer you inpect the car and the more defects you can find, the greater leverage you will have to argue the price down.