They are 1-12 twist. The barrel is similar in profile to the A1 barrel, but they are not interchangable. It will accept the USGI clothespin bipod. It is chrome lined.
Some of the lower internal parts will interchange, or can be modified to work. You could use a highly modified AR bolt carrier. The rear sight windage drum is the same as the M16A1. The Daewoo bolt, firing pin, and most other important upper receiver parts are not interchangable. In a practical sense, you'll have to make something if it broke.
They are post-ban, and fall under all the post-ban BS. Yes, a folding stock would have to be in a fixed open postion. You can't legally convert a post-89 import ban gun (like the DR-200) into a non-importable configuration (i.e. evil features). You can turn it into a domestic gun with US parts to get around that law, but then it's still a post-ban because it had to have been in a pre-ban configuration before the 94 ban to be a "pre-ban". As a US post-ban you are allowed one evil feature. Most people choose the pistol grip. You could go with the folding stock, but then you couldn't have any other evil feature(like a Pistol grip to shoot the rifle with). In practical terms the Pistol Grip is the evil feature of choice.
The thing on the end of the barrel is just a thing. It is not a muzzle brake, flash suppressor or anything useful. It simply permanently covers the threaded end of the barrel, eliminating that evil feature.
If you're anywhere near Virginina Beach, VA you can shoot/examine my pre-ban Daewoo K2 to get an idea of what the gun is like. They are a good gun. I like mine quite a bit, and most people who have tried it like the rifle as well.
Ross