As to weather the cost / value equation comes out on the side of making this degree of investment in a 20 year old Porsche is not for me to decide - the overall condition plus intangible factors of value to you as the owner may add up to being worth the investment.

On any major repair I recommend that you get 3 or 4 estimates from local service facilities unless you are lucky enough to have a shop with an absolutely stellar reputation available. Compare the estimates and choose one of the middle estimates. My reasoning for this is the low estimate either left something out or worse is deliberately giving a low ball estimate to get you in and then when your car is all torn apart they find all sorts of critically important things wrong with it which run your bill through the roof. Do the leg work and be an informed car owner. No reliable technician or shop will give an estimate for repair site unseen except as a budgetary estimate which will be a range from the middle average to the unusual high end possible but because you can not drive around to multiple shops in this situation you are going to have to base your decision on this less than perfect estimation scenario.

You’ll have to have your mechanic take a look and then he will be able to give you an estimate of cost of repairs, and whether or not it’s worth fixing.

Theory. They abound. The reverse band is working, but the pumps to go forward have failed. Or any of a number of other transmission problems.

You broke an axle (although then it shouldn’t back up either).