Would you buy a 12-14 year old Integrated?

I bought a used MA6500 a few months back from Audio Classics.  I have no idea where that would be compared to the 6900.  Make sure the amp has the autotransformers as those are reason for the full and lush McIntosh sound. 

I have a 16 year old Redgum integrated and for the last five years that I've had it the sound is exactly the same. Well, that was wrong, the sound is better because of upgrades in other parts of the system. I am going to send it to Australia for a check-up and to replace the input selector switch that the previous owner played too much with. I expect it to last trouble free for another 7-10 years.
I don't know McIntosh, but if 7900 sounds better and you can manage extra $800, that would probably be the preferrable choice. Unless for whatever reasons you want the 6900, of course.
I also have 18 year old Audiolab integrated that I no longer use, just play it for a few hours every year, the rest of the time it spends in the closet.

I just bought a 14 year old Portal Panache (I had one some years ago) and it sounds great...

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.