Z-box mimics ZO much better on dbw than non-dbw. If you stick w/the GT-40 (non-dbw) the Z-box will be servo controlled. Will it come closer to the ZO feel than Stargazer or Classic? Yes, but it will not be as close as USA1's Moomba w/dbw. You may not be happy w/it.
Best options IMO would be to repower or look for a newer used boat w/ZO. My thoughts are if you can restore the '99 w/new interior, cables, engine w/ZO for $16-18K it is probably worth it. If it will cost $20K it is less attractive and anything over $20K I'd lean toward spending a little more for a newer boat. I've always used the $2K per year rule of thumb, i.e. if you can step up to a '07-08 for less than $2K per year you'll come out ahead. So, if the '99 when all is said and done costs $20K and you can get an '07 or '08 for approx $28K you'll come out way ahead on future resale buying the '07-'08.
Obviously, this rule doesn't take into account budget. If you've only got $20K for a ZO boat you're option is the '99 (and it is a good option). BTW, I've put over 2K hours total on 4 different '97-01 Ski Nautiques and over 500 on an '07 hull. From a slalom perspective I have no preference on hulls. Both are incredible. Handling I'd give the edge to the '07 strictly because the off-side turn (starboard) is superior. Tricking goes to the '97-01 and jump I have no opinion as I don't jump (except when my wife speaks).