If you buy used, does it need to be checked out and calibrated? Probably not just for good/nogood. They do make high volt DC, so a 50 year old tester should be checked out for PS caps and correct voltages.
Since you've already built Transcendent amps, I assume you know about their tube tester kit.
Some newer audio tubes were not common during production of vintage American tube testers so you need to know the settings to use when testing those. I found it difficult to find settings for 6H30. EL509 may fall into that category depending on the model tester.