I've been shooting handguns for a while now and am interested in rifles.  I don't know a whole lot about them but am interested in an AR style rifle.  I'm just curious what some of you more knowledgeable people would recommend for me.  This will be used primarily for home defense and range use.  Also, I'd like to keep my costs under $700 if possible.  

I built my own for less than $650. Just buy a stripped lower, follow the instructions on this site, and buy a kit from sombody.

I've read a bunch about different lowers and which ones people like most.  I haven't found that much info about the uppers.  What should I be looking out for there?

Start off with a RRA lower they are good and dont cost a arm and a leg. M4 profile is great for home defense unless you want a Short barrel under 16" you will have to get it registered  as a SBR= $200 more for the tax stamp. there is alot for sell in the EE.

A number of people here recommend a complete rifle for a first-timer: there's a fair amount to learn and why subject yourself to all of the headaches of troubleshooting a weapon you're completely unfamiliar with?  You didn't build your first semi-auto pistol, did you?

A good new AR will run you a bit over $800 and that will come with a warranty from the factory.  Something to consider.  Bushmaster and Armalite fit the bill.  Colts are sought after but they run a couple of hundred bucks higher.

My two cents, anyway.

I've got a Bushmaster XM15E2S (A2 style carry handle and 20" HBAR barrel).  It's a bit heavy for room-clearing chores compared with newer AR15 designs, but I like the ability to mount a bayonet.  Pistols may be better suited to room-clearing than a full size A2 style rifle.

 Perhaps a flat-top style rifle, with a 16" barrel and a "shorty" fixed buttstock or an adjustable buttstock would serve you well from zero to 300 yards.  Add optics, flashlight, laser, and/or  iron sights per your tastes.

 I suggest "test driving" and researching various rifles to determine what feels comfortable for you and what will meet your minimum requirements for target incapacitation.

Good luck.
~Craig