I'll throw out a few definitions, understand that I am also a noob and might require slight correction.
PDW = 7.5" barrel. Noisy as hell to the shooter. Very high pressure in the gas system.
CQB = 10.5" barrel, or close. I think usually has carbine length gas system, still high pressure but more common. In fact, I see more parts for carbine length gas systems (buffers & springs, receiver extensions, stocks) for carbine length than any other, by a wide margin.
Carbine = 14.5-16" barrel, carbine gas system is usually 7.5-8" down the barrel
Rifle can have either midlength or rifle length gas systems (I don't know the exact lengths), which are lower pressure so take softer buffers and springs. Typically, what, anywhere from 18" - 24" barrels are common.
You can mix and match uppers and lowers most of the time, though some manufacturers build to their own spec and may take minor machining to match with other brands. There are gazillions of upper, lower, and parts manufacturers...the guns are mostly plug and play with different parts.
Whatever you do, don't ask who makes 'the best' of anything...you'll never get a straight answer
$3500 will get you the top end of anything available in Canada, so just pick your wishlist parts or look and go from there.