Well as you read through threads here I'm afraid you'll see a fair share of negatives concerning DPMS. Some people have good luck but quite a few don't. To offer a low price DPMS cuts corners, for example the non-milspec receiver extension that you mentioned in #3 which costs a few bucks more than the commercial version (btw, you don't have to replace it but you do have to be aware you have it if replacing the stock - as far as the buffer and spring, wait on that and shoot the rifle with various ammo to see if it works ok).
It is not milspec since it's not built to military specifications. But it may work ok for average use however you may have to tweak a few things. I'd say get out there and shoot it before making changes. Find out what you really want after getting at least a few hundred rounds down range. Also expensive upgrades on an inexpensive AR may not be to your advantage. If you decide to sell it and buy something better later you won't get back a dime of the money invested in upgrades.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek