No need for a crazy expensive crystal, presumably you are not looking for neutrinos. what are you using as a power supply? I have a kiethley tube deal but am swiitching to a modern compact spelman. You know what part of the design I spent the most time agonizing about? Deciding on a connector for the HV line... I went back and forth between BNC, MHV, and SHV and which type of coax a dozen times. Dogshed in my head.
Incidentally, it seems that shv connectors are often misidentified as RP-BNC by those that don't know better, you can use this to find deals on them.