I think 1/4 is fine, unless you have a panel that is more than maybe 2 feet by 4 feet.
My doors were not sealed before assembly. Have not had any issues. They have been up for a year or so.
strange, everyone around here has 1/4 1/2 and 3/4 MDF in stock. One place even has different densities of each, plus 3/8, 4x10 and 4x12 panels.