Okay, I am no expert, nor do I play one on line.
I have never dealt with the AR-Parts-Kit-Incorporated that you linked. One thing I noticed is that while their prices go up as you add options, their prices on the upgrades are not THAT bad given the kit.
YHM CAR solid 4 Rail forearm tube (+$114.00)
Most of the 4 rail systems from Yankee Hill Machine run in excess $130 dollars, plus shipping, plus you have to install them yourself. So in that instance, you are getting a deal.
With Badger GEN 2 Tactical Latch (+$20.50)
Badger Ord makes one of the best tactical charging handles for AR's in my opinion and it's going to cost you $20 plus shipping to get one, so this isn't a bad deal.
Magpul CTR Stock add (+$99.00)
Great unit: It's going to cost you $90-$100 plus shipping IF you can find it in stock anywhere, plus you need the tool to install it correctly - so you make the call.
So, I don't know if they are giving you the same quality, correctly assembled, correctly timed, correctly headspaces unit that we would give you from our shop - but they aren't outright gouging you either...
So, take that information for what it's worth. Building your own is ALWAYS the best way to go, even in these trying times....