the LT should be fine in 12 knots
with your 80 kilos / 176 pounds and an 8.x (8.0, 8.5) meter sail you should be flying/planing
most windsurfers I know have a board quiver of two to three boards
and probably more than three sails
where i am, there are more and more kites and less and less windsurfers
people with longboards even less
i can take my Mistral Equipe 2 XR out in 10 - 20 knots no problem
no wave jumping, but not in danger either
15+ knots is where the fun starts
it all depends on location and your availability to go when there is wind
perhaps you can rent or try some equipment in that 125 to 135 liter range ??
but no matter what, priorities are always safety and then FUN, FUN , FUN !!
welcome back to this great sport !!
(sometimes a calculator helps a bit --
jimbodouglass.blogspot.com/2010/11/updated-windsurf-calculator-online.html)
even if i suck at windsurfing, I can still have lotsa fun LOL