As I understand it:
170mm rear ends and offset rear ends like the Pugsley need a 100mm BB into which a crank is fitted that has the same arms as a normal 68/73mm BB crank but with a longer axle. 190mm rear ends use a crank with a straighter drive side arm to move the chainrings out a bit to clear huge tyres.
Q factor is always big on fat bikes. It may affect you and your knees or other biomechanics, or it may not, I actualy find it solves my heel rubbing problem on the chainstays I get on most nomral bikes.
The On-One Fatty isn’t the lightest or the best spec’d, but anything better rapidly get’s to a lot of money even for Chinese Carbon (the rims are $280 each + shipping even from light bike) and I wasn’t prepared to spend that sort of money if it turned out not as expected. That and it’s a known quantity, it has quirks (getting the tyres seated is an art), but at least the chain doesn’t rub, all 20 gears work, the rear wheel is in line with the rest of the bike etc, all things you read about more expensive custom builds falling over on.
I’d not go chinese custom unless you have ridden a few and know what you’re doing, the Fatty rides nothing like a skinny wheeled bike if you think of the numbers, and I imagine chainrig clerance etc are going to be difficult to get right first time. Although Shand etc could do a good job if you do want to spend that much on a first bike.