I would be concerned about the evaporator if the AC was just charged. The rumble at idle could be the top motor mount, or torque rod bushing, as it is more properly called. They tend to fail regularly and when you replace it with a longer lasting urethane bushing, it transmits more vibration to the firewall. I sensed this on my 850 when I went to a urethane bushing. The urethane bushings are typically a funky color like bright blue or yellow.

Of course, if you were serious about the car you should have it checked out by a Volvo specialist. $7,900 seems like a lot for a '94 855T with 135K. I have seen postings of 850s with close to 300K on the clock. Good luck.