I'll probably buy used, but i need some direction.
If that's the case, perhaps you should buy from a LGS or private party where you can inspect and perhaps even shoot the gun. This is even more important if the gun has been assembled by the owner or seller from parts and is not a complete "brand" gun... i.e. lower is one brand, upper (which is seldom marked) is another, barrel yet another and so on. Buyer beware.
Safe bets for a guy on a budget are new guns being sold at discount prices from your local Walmart, Fred Meyer, what ever big box you got... etc etc. New, name brand rifles can frequently be had for $750 and less. At least you get a warranty from the Mfg.
You want a barrel marked .223/5.56 NATO or 5.56 NATO or .223 Wylde... not
just .223 Rem. For twist rate a 1:7 has the broadest range of use, while a 1:8 or 1:9 will have slightly less heavy bullet capabilities. If heavy bullets aren't your thing, then 1:9 is nice.
I'm not going to start rattling off brands... there are too many good ones to choose from, especially entry level guns. A removable carry handle or flat top is the preferred upper. Barrel length and stock type are your call.