Absolutely conceivable that a leak stays at the same level. That's a pretty big leak, so I can see that it doesn't get worse at some point.
The 964 case should be the same as the the 993. The seal is easily removed and re-installed. I'd recommend an installation tool, which can be borrowed, but a piece of PCV pipe and a rubber mallet do as well.
The issue is the location of the seal. To get to it the flywheel has to come off, which means splitting trans and engine. Is that what the mechanic meant? 99% of the people drop engine and trans. Some also remove just the transmission - which I find daunting unless you have a lift.
Get the shop to quote the RMS repair and take it off the asking price. Then drop the engine yourself and install a seal that's less than $20. It will be only your labor!
G