Installing AC in a car that did not originally have it is going to require a lot of labor. The parts should be fairly easy to acquire from a wrecked car, but the overall cost would be enough to cover a complete overhaul on the '99.
I don't like the interior color combination of the 99. That red and grey just don't like to be in the same car together, but those are actually very nice seats. I'd probably trade the seats for ones that match better.
Unless the 2.7 has a low enough price to compensate for installing the AC, I would choose the 2.5.
...and I own a 2001 2.7 and a 1999 2.5. Love them both.
