12mm should be ok lah.
jumping from 12mm to 15mm thickness is not $1 to $2. the jump is very high. for my crystal glass tank, 12mm to 15mm is almost double the price.
so do consider. but for safety reasons, 15mm is definitely better. but 12mm should be fine. my tank 3.5 x 3.5 x 2 is 12mm.
as for turnover rate, not sure. 6x is minimum but i would go for at least 8-10 times if you can.
the higher the turnover rate the better.