At first I planned to epoxy coat the inside of the stand like you said, but in the end it was easier to just paint the stand and do the tray, plus I would assume it will hold water better if there is ever a need.I like how the sump matches the stand. I brute forced my stand out of 2x4s and 2x6s, but I'm going to put solid wood frame with removable raised panel doors.
I really like the idea of having a tray underneath the sump. I might end up doing that instead of epoxy sealing my entire bottom plate