On the whole, what you are told is probably pretty average. As long as the valve springs were checked replaced at 200-250 hours, you should be able to get 500 out of the whole package before going real deep. But that all depends on how hard they have been run.
Try to get it on a computer/scanner and see what the RPM spectrum has been for the engines. If they have 150 hours of idle time (some people have REALLY long no wake zones, esp. in Florida), they might be healthier set of engines than you think.
Use the rebuild costs as a way to set your offer price.
We have an AT owner that has over 900 hours on his 37AT and he past the 700 hour mark before he had to go deep into the 500s.