Ok, first off, it would definitely be a smart idea to go with the Stereo Integrity 18HT or the Dayton HO18, so you are good there. I like and use the iNuke3000 in my own theater, only thing is I would suggest that you spring for the extra $50 to $75 and get the DSP version aa it will allow you to set a high pass filter, EQ, and some other nice features.
As far as enclosures go, you could get by with sealed as your room is small, but, a pair of ported 18" enclosures would be the ultimate in terms of bass output, and extension I'm your sma room! I would definitely recommend the ported over the sealed if nothing else due to the fact that it will net you 10db to 12db of additional output around the tuning frequency, and have much more slam and the "hit you in the chest" type feeling, compared to a sealed!
As far as the enclosure goes, you need to learn to model the different enclosures in a software program such as winISD, although that is kind of like the pot calling the kettle black as I don't know how to use it either since I do all of my internet surfing on my iPhone 5s.
Contact member LTD02 and tell him your size limitations and. I am sure he will model the best solution for your particular situation! We also have members that will build your enclosure into a flat pack kit, for a nominal fee, of course!