First, the answer to all your questions is, for the most part, yes. You have three basic options for sub builds.
First, DIYSG and GSG offer pre-cut, pre designed sub cabinet panels. You'll pay a small premium for them to design and cut your boards but they go together perfectly. You may, however, pay a lot in shipping costs. Also in this camp are finding a cabinet shop or car audio place to build a cabinet for you. I don't see these as a necessary nor cost effective option for someone with your carpentry skills.
Second, if you have some skills and tools you can grab existing plans and build your own boxes. The Marty variants are very popular, as are the VBSS's. As a master carpenter these would be an absolute breeze for you. My carpentry skills suck and I managed (with a bit of post-build material removal and wood filler).
Third, you can design or have designed very specific custom subs designed just for you to accommodate your exact space considerations. Folks use computer software to compute box and port size to match specific drivers. Then, you build the boxes.
In general, bigger boxes and bigger drivers give you more output and lower tuning options. Also consider that you can't really have "too much." You can always reduce the gain on a sub that gives you more than you need, but you can only turn an inadequate sub up so high.
That's the box part. The other considerations are drivers and amps. For super-budget builds many folks use the PA460 driver in a VBSS cabinet. You can find the driver for $80 (I'm building a few now and they come to like $105 per assembled box!). UM18's are very popular too. When on sale you can grab them for $250. Both of these drivers marry nicely to the inexpensive Behringer dsp amps. Many on here have moved to 21" drivers from LaVoce and B&C. I have no experience with those, but the drivers are a step up in price and some use significantly more powerful amps to power them, which may have cost and electrical circuitry considerations, but I'll let the 21" driver guys chime in on that.
How many do you need? To answer that question you need to know what you have with regard to room acoustics. I would recommend this as an absolute requirement:
Grab this $75 mic and download free REW software. You'll use this to figure out how many subs you need, where they should go, and to calibrate them once you put them in place.