Would you buy a 12-14 year old Integrated?

Go for it!   12 years old it should be fine!

Amplifier design has matured a long time ago, so new is not necessarily better. I have three power amplifiers. A Quad 303 from 1970, a Quad 405-2 from about 1992 and a Quad 606-2 from a bit later again. All sound absolutely fine, even if the bigger power output of the later models gives a cleaner sound with more dynamic music. And all have been completely refurbished. The important components here are the large electrolythic capacitors. These do degrade with time, and that can make for a slightly less dynamic perfomance. Most importantly, however, if they fail you may have an expensive disaster on your hands. So I would insist that these are replaced. Doing so every 10-15 years is good practice.

No. Aging caps is a serious problem especially recently where manufacturing moved from Japan to China.

Bryston b135sst2 (if you can find one). 20 year warranty. Need I say more.

Yes especially from a top brand. I could rebuild it 
WITH MUCH HIGHER QUALITY PSRTS.