There is very limited service information available on these. Service was basically by board swap. If you are very good, you might be able to troubleshoot it.
Does it have the logic pod? You can spend up to a couple hundred on that if it doesn't come with the scope.
There were also piggyback modules for that scope that added fft and gpib or serial comm. Does it come with any? Those can be $50-150
I have one that's all tricked out and I like it. But I would probably go for one that doesn't have any problems or more likely a 54622d. The 54622d is around the same price but is the 54645d replacement and the firmware can be updated.