First decide what you are going to be doing the most with your firearm, target, varmint, home defense, long range, match shooting, etc. Decide on what weight you are comfortable with. This will help you decide between some of the lightweight upper/barrel assemblies versus target bull barrels. As for barrel twist, a 1:9 twist is almost the norm anymore. However, if you are going to be shooting some of the heavier grain ammunition, you will want a faster twist, such as the 1:8 or even the 1:7. A flat top upper, A3, will make mounting any opticals alot easier, but there are also mounts for the A2 handle as well. Also, if you are going for accuracy, most experienced shooters will tell you, the A2 (solid) stock is preferred over the retractable stocks, as they provide more support and a better cheekweld. But the retractable stocks are adjustable in length for what size of shooter or to compensate for heavier clothing, etc.
As for buying separates versus a complete gun, it's a toss. A complete upper will run you about $350 and up depending on what all you want. A complete lower will run around $250 and up for some of the lesser brands and higher for a Bushmaster, again depending on what all you want. So just from there, you can see that you've started at that $600 mark. But...know that you can quickly go past that limit. Complete Bushmasters, look at AIM Surplus, as they sometimes have them on sale starting about $675.
Go to the manufacturers sites and take a look at what they have to offer, go to a local gun show, gun shops, etc. Some gun ranges have rentals so that you can try one out to see what you like and don't like. Don't necessarily be swayed by polls, forums, etc., use them for reference if you like, but the final choice should come down to what you like and can afford, not what the latest arm-chair commando-type is carrying.