2001 Infiniti I30 INVEST OR TRASH?

I purchased my 2001 Infiniti I30 new. It has about 118K miles on it. The air stopped working so I took it in to a trusted mechanic who told me there is a leak. Apparently there are several issues...I need work on the evaporator, condensor and also some kind of idle control. Estimate: $1700. My mechanic told me I'm better off buying a new car since the next thing to go could be the transmission. I'm at a crossroads. I don't know whether to just do what it takes to fix this car (which has been a great car for almost 10 yrs until now) or listen to his advice and move on. I'm not crazy about investing in a new car right now but at the same time, I don't want a money pit either. I just spent $500 replacing the alternator already.

I'm not sure what the mechanical history is of the Nissan engine with this many miles but my mechanic seems to think they have transmission problems. Any insight would help me make this decision and would be greatly appreciated! ~Erin in FL