for me the most common used sail is my 8.5
but it depends on your weight.
if your over 85kg i'd say 8.5, 7.5, 6.6
if your under 85kg i'd say 7.5, 6.6, 5.8
if your under 75kg 7.5, 5.8, 5
all of the stuff above is for speed and slalom.
for boards you would want your weight + 20 for light winds and something smaller for when it blows. there are some excellent speed locations up here.
normally we get 10-15 knots. lately it has been awesome 20-30 knots regularly. i've been overpowered on a 5.3 at times. so i haven't rigged the 8.5 for about 3 weeks. we seem to be having a windy season so far.

vando's the man to talk to about the ins and outs of wello and manly. he spends more time on the water than anyone and it is rumoured that he actually is half fish. he knows the place like the back of his hand. vicki point is my backyard so i go there regularly or down the gold coast. the train is another spot for speed freaks.
the speed spots are
the train (gold coast), thomsons beach (victoria point), Wello (wellington point), lota creek (manly), golden beach (Caloundra)
not quite sandy point but pretty good none the less.
it's sad to hear your lakes are drying up. that can't be a good thing! lots of the guys around vicki point and redland bay ride trail bikes so you will find some good places for that as well.
oh, and as far as night life goes. lots of pubs around but not much in the way of clubs in the bayside area. so if you want clubs you would need to go to brissy or down the gold coast.