I believe building one makes it easier on the wallet. The nice thing about that is building it will require only minimal assembly skills. I built my PRK legal FAB-10 and I am not a gunsmith by any means. I am going to outline my process as I think it served me well.
1. Buy a complete upper from AR15.com Equipment Exchange.
I got a like new
20" Bushmaster V-Match fluted flattop upper with bolt/bolt carrier/charging handle (commonly known as B/BC/CH) for $425 shipped to my door. As you can see from my Bushmaster link above, that upper is actually $625 plus shipping from Bushmaster.
20" Picture
2. Buy a lower receiver.
Now the lower receiver is the serial numbered part you are going to have to order from a FFL. I unfortunately live in the PRK (California) and had to order a
FAB-10 lower which cost me $323. You should be able to get a real lower that accepts detachable magaines for about $100 to $150. You might even be able to find a lower receiver complete with buttstock and the trigger parts for as low as $250 to $300.
3. Buy a buttstock.
Buttstock sets on the AR15.com Equipment Exchange can come pretty cheap. I would try to spend more than $55 shipped to your door for the buttstock, spring, buffer, buffer tube and other little parts.
4. Buy a lower parts set.
This should be no more than $55. I got mine from
Rock River Arms.
5. Put it all together.
Here are the instructions I used and I did not use any special tools. I found it rather easy to do, easier than I thought it was going to be.
6. Buy a new upper to add to your collection.
A few months after I bought my 20" varmint upper, I bought another upper off of the AR15.com Equipment Exchange. This time a
Bushmaster 11.5"/5.5" A2 (carry handle) complete with B/BC/CH for $325 shipped to my door.
11.5/5.5 Picture. Again it is hardly used and shoots great. I have consistantly saved about $200 from buying from guys in the AR15.com Equipment Exchange. And the best thing is these uppers are not controlled and they can be shipped door to door all across the country!
As far as a recommendation on what to get. I would recomment a 20" A3 with detachable carry handle for now. Once you get that and have fun with it, then start working on a 16" flattop with optics and all of the other fun doo dads like verticle grips, flashlights, flip up back up sights, and collapsable stocks. If you want to skip the 20" stage, then get a 16". I like my 11.5" barrel carbine and 5.5" flash suppresor, but I wouldn't have minded getting a 16". I just wanted it and it was a great price so I didn't wait.
I hope this helped.