Now before you get in way over the OP's head, Tell me how many AR manufactures or barrel manufactures that list gas port size?
Very few makers list or will tell you what the port size of their barrels are. But I can tell you from experience tuning ARs to run suppressed or unsuppressed that the gas ports of the Colts I've played with are on the money.
You did forget to add that the Mid length gas system came out to deal with the short comings of the carbine gas system.
The industry has learned a lot about port sizes, buffer weights, extractor springs and action springs since Armalite developed the middy. The biggest problem with a 16" carbine gas system AR was over gassing and a weak extractor spring. Once the right gas port size was figured out and Colt redesigned the extractor springs, the shortcomings of the carbine gas system have vanished.
The Colt 6920, 6720 and 6960 are out of the OP's price range and are not considered entry level guns.
Colts cost more than the $500 limit armoredman has set. But the 6920 and 6720 are perfect entry level AR carbines because all a shooter has to do is clean off the preservative, lube and add ammo to get shooting. No worries about spring quality, feed ramps, consumer grade parts or extra features or furniture that drive up cost but add no value. A budget carbine that's under-buffered, over-gassed and fitted with consumer grade springs will soon have a shooter new to ARs spending more time trying to troubleshoot their new rifle than shooting it.
What entry level AR carbine comes with anything other then a standard weight carbine buffer? I do agree that the heaver buffers do work better on most carbines. A heaver buffer is a good upgrade that will cost you $30 to $50.
Colt civilian carbines come from the factory with H buffers. Colt military carbines have H2 buffers. The original buffer is the 5.3 oz rifle buffer. H and H2 buffers are not heavy buffers. They are the
standard weights for M4 carbine style ARs.
An entry level AR isn't defined by cost. The entry level AR is a basic carbine or rifle that gives the shooter trouble free operation and good value for the dollar.
Let's say armoredman orders the PSA rifle in question. It will cost as follows-
-$449 for the rifle
-$25 for FFL fees
-$23 for a PSA H buffer
-$55 for a BUIS
-$25 for springs (PSA extractor & action springs tend to start failing after 2 or 3 hundred rounds)
-$30 for 2 Magpul 30 round magazine
-Total $607
That's not bad, really. But Freedom barrels do tend to be over-gassed and deliver 2-3 MOA precision
But what if he buys a 6920 with Magpul Slimline furniture from G&R Tactical?
-$919 (after discount code) with two Colt 20 round mags and Magpul Buis
-$10 shipping
-$25 FFL fees
-Total $954
That's $350 more. That's $350 that could be spent towards ammo, or pay for most of an Aimpoint Pro. But that's also for an AR that's already correctly tuned, that already has the right buffer and springs, that won't start having extraction problems after a few hundred rounds. That's for an AR that has all quality in spec small parts.
I had a PSA and after sent the upper back to have the feed ramps cut right, replaced the underweight buffer and worn consumer grade springs, it ran great. I loved it. It came with a Wilson stainless steel barrel which gave good enough accuracy with blaster ammo. But I didn't have to do anything to my Colt 6920 to get it to run right. I'm not planning to ever sell the Colt, but if I do, I'll recover a higher percentage of my investment than I would by selling the PSA.
In the end, armoredman is going to have to make up his mind for himself. Pay $607 for an AR and the needed upgrade parts and likely has an over-sized gas port, or $954 for a Colt that he can rely on right out of the box with an action that has been subjected to years of refinement. It's also possible that with careful shopping, a couple hundred dollars can be shaved off the price of the Colt.
Why is a mid length gas system desirable as opposed to the carbine length? Why the nitride finish? Why new take down pins? I really don't know any of the questions to even ask to begin with a new rifle. My service rifle was an M-14, for crying out loud. 
