[Porchdog]

No direct experience with modern DPMS - older ones were hit and miss. With all of the competition in the market today I would expect them all to be solid.
Nothing wrong with the PSA guns. The higher specification barrels are worth the extra money - their bargain barrels can be hit and miss as well.
I have had a Rock River gun for a decade and a half that has been a solid performer and very accurate.
There are plenty of AR's that just work - I don't know what it takes or which manufacturers sell into Canada.
id10t is correct - there is nothing to putting an upper and lower together and everything is interchangeable.
One difference you may have in Canada is taxes. Here in the US the tax on "firearms" is very high - but not on "parts", So you can buy a lower (which is serial numbered and thus taxed) and a "parts kit" for a lot less than a complete gun - thus the popularity of doing that here.
DPMS, PSA, Smith and Wesson, Bushmaster, Windham, Stag, Spikes - All make good entry level guns. Pick the features you want and the price you are OK with and don't worry to much about the brand.
If you want to change anything about the gun down the road it is easy - one reason AR's are popular is easy interchange of parts.