1. Is sealed the best at meeting my requirements? If not, why not?
Not in a room that size with the box size limitations that you have. A ported sub would have more LFE and SPL then a sealed sub. The down firing cabinet would have a net volume of 5.2 cu.ft in a ported configuration. A Shiva-X in that size cabinet tuned to 19 hz with two 4 inch ports 33 inches long would give you the most output for the money. If you want to go sealed, you can expect a lot less low end output.
If you mean the EP2500 from Behringer, then yes, for most applications. Note that depending on what sub you choose and what cabinet type, you may also need a High Pass Filter.