The M103 engine is known for bad valve stem seals. However, it was the earlier models, like the '86 and '87 in which this was a real problem. The possibility exists that valve seals could bring the oil consumption to a workable level. The most likely fix is a valve job including valve guide replacement. If you have this done at an independent shop, you are probably looking at around a $1,500 job.
The rear window, since you can hear the motor, needs to be investigated by removing the door panel, but it will probably require a window riser.
Best of luck,