If it is shuddering continually while the transmission is in reverse, it could be motor mounts, but it also could be something transmission related. Follow up on this with the mechanic.

Also, while there, ask the mechanic if he can look at the a/c system to determine if the compressor is shot or not. Probably cannot tell for sure, but, might give you some idea.

If the suddering is not the transmission, the climate control works properly otherwise, no issues with the engine, and the a/c does not require a complete rebuild of the system, then you have a decent deal for $3,000.

If you have to dump a grand into the a/c, then you would be advised to have patience and wait for a better one to come along. $4,000 for a vehicle with that mileage is too much, IMHO.