Hi,
Most liveabords to the Similans take 4 days, some do also 6 days. The 6 days ones are not the budget trips though.
What you could consider, is making a trip to the South Andaman (Hin Daeng/Hin Muang), followed by 4 days Similans.
For a 4 days Similan trip you have to count on appx. 20,000 Baht, for a nice boat including AC etc. Cheaper boats are also possible, but only on basic (but safe) boats.
Don't wait too long, because there are lots of pre bookings this year, and you guys are with a big group.
Steven