I don't think Engineer should forget mid volume board, because jump to a 90 l might be a bit too much to handle (for a Go sailor). He's only started to use harness, so not that much experience.
Engineer, try to find second hand Carve122 or 111 and try them. Both will carry a 7.5 well - for your lower wind range. 111 will be hard work in the beginning (as Leysenkr says), but more fun later. (I know whose forum is this, but there are similar boards to consider: rocket, eagle, stoke, xCite, etc if you can't find *board)
For 25-30 knots you'll need 78l kombat or smaller, too early to decide ;-)