As the Model01 manual puts it, all keyboards are bad for your hand health, but ergonomic ones are in most cases, much less bad than other keyboards. I’ve been using ergonomic keyboards for quite a few years now (ErgoDox EZ first, Model01 later), and I can’t imagine going back to traditional keyboards. Love them dearly.
Yes! It’s one of - if not THE - best keyboards on the market at the moment. The thumb arc, the palm key are all amazing. The fully sculpted custom keycaps likewise. And the halves having standard camera mounts allow one to do some very interesting tenting setups:
I have a Quiet Click, and it’s my favourite switch. It is quiet, more quiet than the cheap membrane keyboard I used to use when I needed to silence the clickety clacks. I love the weight of the switches too, they’re not buttery light, but they don’t require hammering to activate either - it’s a sweet spot between the two. I wouldn’t mind a bit more pronounced tactile bump, but that’s about it.
Depends on the game, really. I’ve played Elder Scrolls Online with it fine, worked amazing for that, and plenty of others. My main issue when gaming was that to make game controls feel good, I had to either remap the left half so it had all the shortcuts I wanted, or I needed to remap the key bindings in the game. Remapping the keyboard has the disadvantage that if you want to type, you’ll have to switch to a non-gaming layer. Remapping in-game is mind numbingly boring.
But then, this goes for any kind of split keyboard. If you have the patience to set it up to fit your needs, it works very well for gaming.
Oh, and the keyboard is fast (unless you sprinkle expensive LED animations onto every key press, that will slow things down).
Matias Quiet Click, mostly. You can find Loud Click variants for sale here and there, but in general, the Quiet Click variant is the most common. These are ALPS-style switches, not compatible with MX-style keycaps. While the switches themselves are compatible with ALPS keycaps, the physical layout and the frame of the Model01 is not, you’re pretty much tied to the custom keycaps. But those are the best, anyway. 
You might be able to buy a used one, yes. Buying from the shop, it ships from Hong Kong, I believe, so import taxes and whatnot apply in that case. (Those should be included in the price at checkout, though, so you shouldn’t be paying anything on top of that.)