Is the mtr on a boat or sitting on a rack ??
If you can, do a compression check on it along with checking the gear case oil for any metal particles or water. Then take it for a good test run.
I know they have Blue Books out there for marine mtrs but I don't know where. Maybe somebody who knows will chime in.
I am looking at a 2000 Johnson 150 with controls.. what would you expect to pay for it?
In mint condition with wiring harness and instruments, I paid $2,300 last October. I have no idea as to whether that was a good price or not.