Some people have their presses on here for that long or longer and seem happy. Mine is new, so I'll let them elaborate on that so you get first hand info on that.
As for the warranty.... that is what I want to talk to you about. Make sure the warranty is transferrable to you. Sometimes, the warranty is only for the original purchaser, and in the case of Mighty Press, I think that is the case, double check.
If the owner does not have the manual, call Mighty Press (Stahls). The info may be right on Stahls website, but if it changed over the years, you'll need to know that.
If it is a good price or not depends on how much they go for new, with a full warranty... compared to how good of condition this one is in, and if it comes with the warranty or not.
What to check: I would be prepared to go there and spend a good half hour there. Don't let him warm it up before you get there. Time to see how long it takes to heat up to full heat. Mine is about 10 minutes (Mighty Press Lite). Once it reaches the temp, the green light goes off.
Next, bring a cotton tee shirt and transfer paper. Cut the transfer paper into about 9 boxes. Place those boxes spread out on the tee shirt, so you can press them, and see if they all press the same, with the same results. If they do, you know the upper platen is heating evenly across, that is a great sign. If you get varied results, some stick and do not peel, some look bad, etc. then there may be an issue with the heating element. Realize tho, for a hot peel, as the paper cools, it gets harder to peel (usually), so reheating real quick (10 seconds or so) to peel the second half of boxes might not be uncommon.
Then after about 20 minutes, do the test again. See if the results are the same. If they are, it is a good sign the heating element is able to hold *consistant* heat without flucuations. That is important.
Another thing you can do is bring an IR Gun to check the temp on the upper platen to see if the heating element is in sync with the temp control.
Look over the electrical cord and attachments. Check the arm and pressure knobs, make sure they function and are sturdy. That's all I can think of off the top of my head... good luck...