The BW65 is a bit stronger than the BW35 but side by side the 65 is a little smaller. It can certainly stand up to more power than the 35. As said it was used in the Jaguar XJ6 for a while. I had a 65 behind an Triumph 1850cc engine and the only problems I had was a spell of reluctance to change up at part throttle. This was found to be dirt in the fluid which was blocking the valves. After a rebuild it was fine and I was glad the car was still under warranty.
Check the fluid in the current car and the "for sale" box. It should be pinky red (depends on fluid used) but shouldn't look black or smell burnt. Burnt means the clutches are on their way out so make sure the "new" one is OK. If the $50 box seems good it's worth the money to wrap it up and keep until the other one needs to be changed. Better if the engine is with it.