99 Sable trans/speedometer

I have the chance to buy a 1999 sable. The problem is the speedometer isn't working and the transmission is shifting funny. It will shift and it seems kind of hard but the engine revs up before it shifts. Also the o/d light will come on and the service engine soon light is on. I'm going to take it and pull the codes this morning. My question is this, everything is pointing to a vehicle speed sensor. If that is what it is can driving on the bad sensor hurt the transmission. I called my transmission guy and he said that the speedometer not working is probably the speed sensor and that would need fixed before you could tell if the transmission is any good. I don't know how hard a VSS is to do but he said it would be between 250 and 300. I'm not putting that kind of money in a car that's not mine. I'm looking at right at $1000 (130000 miles) for the car. Its at a chevy dealer and because it is a ford they won't even look at it. If it's just the VSS I think it would be a pretty good deal. The last thing I want is to buy this car and it need a new transmission. Any advice?