Actually Syn,  Your questions are good ones not normally thought of by many builders. To address the marine grade plywood vs. mdf question, I will tell you that the mdf wins. Not so much about quality/price, but due to the fact that a bass speaker is better off with a stiff cabinet. This allows less resonance. The only sound product leaving the cabinet should come from the speaker and not the cabinet itself.  Question two; The type of finish used in such a cabinet is moot due to the frequencies being produced, i.e. they are low frequencies and even their related harmonic frequencies are not of an amplitude that would make any difference as far as finish is concerned.
 Q: I would not doubt that thicker is better, but remember to adjust the measurements accordingly. If the thicker material is on the outside of the cabinet only, this would not apply. As much a s anything is that the supports/braces are well glued and fit well. I like to use various glues during construction. Polyurethane for all outside panels, and tightbond III for the rest.
 Lastly, as in any good design, it is important that the pieces are cut accurately. You probably have good tools and a understanding of this. When complete, I would be especially interested in the outcome.