It could be done, with corner placement in a 10x12 foot room. But that's not how speaker SPLs are measured. It's like claiming a 350HP V6 car can get 80MPG...driving down a 30% grade with a 50 MPH tailwind.
Always avoid the main crossings. In most places there are two, one on the main highway, and another in town. The in town crossings are where the locals go, and the guards on both sides are a lot more laid back. Finding the in town crossings isn't easy, they're not publicized because they don't want everyone flocking to them, but you can find them on line with some work.
Learning how to use box design programs like WinISD Alpha Pro is the first step in learning how to design speakers. It's the only way to be able to accurately predict how a given driver/enclosure/room combination will work without wasting lots of time and plywood. Until you get up to speed with it others may help you out with some calcs. One thing you want to do is determine if you want to go ported or sealed based on room dimensions, then choose the driver based on that,...
I doubt it. One of the facts about DIY is that you can't compete with the buying power of manufacturers for materials, so you won't save compared to factory made at the lower end of the price spectrum. Where DIY has the advantage is in labor, so it's with labor intensive designs that you can save big time. That means you can't make a $100 sub for less, but you can make a $1000 sub for a lot less.
There's also the matter of all the additional electronics probably used. There's a reason why Bose retailers will never have Bose in the same room with a good system. Read this:http://web.archive.org/web/20130121081646/http://www.intellexual.net/bose.html
What do you have for mains? There may be no advantage to having a sub based on what they are. OTOH if you are going to have a sub don't go small to keep from bothering the neighbors. That's why subs have volume controls.
Model them in both sealed and ported and see for yourself the difference in response and maximum SPL. For the most part if you have a smaller room, no more than 20 feet in the longest dimension, cabin gain will aid in the first octave and you can use sealed. In a larger room with no cabin gain you either need to go ported or you need to use at least twice as many sealed to get equivalent maximum SPL below 30Hz.