GPUs should last a good while - outside of just electronic failure that happens. I've had HDDs, DVD players, VHS players and so on die after 6-12 months. Whereas I've had HDDs last 7+ years and I still make use of them, GPUs that were used/abused for 6-7 years and still works to this day. I had two GTX 280s I ran in SLI for about 2.5 years. I ended up going to a GTX 570 SLI setup and gave one of the GTX 280 cards to my younger brother. He used it for 4 years (never cleaned it out - the dust on it was so thick!) and it still works to this day. Once I got the card back I spent 30 minutes cleaning it and applying new TIM. I then picked up a 980Ti (still using it) about 1.5 years back and gave one of my GTX 570s to my brother. I used it for 4.5 years and he has been using it for 1.5 years and she's still going in his computer today.
If you need more power, the 980Ti is the way to go.
But, as others have said, you're buying it used. There's always possibility of any piece of electronic equipment failing (regardless of age). In your case, you would have no warranty coverage on it unless your work partner is okay with setting in a RMA under his info (since he purchased it) for you should it fail.