Just adding my $.02
Glassed over centerboard wouldn't be a deal breaker for me knowing how the boat sails without it down.
If a thorough survey goes good, don't let the centerboard thing stop you. Did you ask the PO why they glassed it over? Maybe it could be undone, for all we know its because they didn't want the maintenance aspect of it? Worst case, its because the penet pipe has failed or something structural?
I am not familiar with the T31 first hand so add much, but I would think the additional size would be most appealing