If it sounds good it is good. With those cheaper amps you may not even keep doubling power as you halve impedance, and halving impedance quadruples the current/heat that its semiconductors have to deal with, so overall I don't think you're missing too much.
Expect any amp to fail prematurely at .5. It is absolutely torturous on semiconductors. You should never do it unless you can afford the cost and downtime of blowing stuff up.
Same here.
B- stock Sundown should be fine, my beef with the new Sundown is that softparts are proprietary and expensive if you break it. Also double check mounting depth on those and make sure you can even fit a 15" X in there.
There should be no audible difference between any of the popular Korean built 2K amps. Pick whichever is the best deal, the best customer service, or just whoever has a logo you like... they're pretty much all the same.
Building a box isn't rocket surgery, if you have some spare time and a place to work you can probably do that yourself. If you can't find a reliable shop, try to check out some local competitions and see who does good work or simply buy some plans and take them to a proper cabinet shop and ask them to knock it together for you. If you explain what needs doing I'd think that sort of shop should be able to make it happen.