It appears that you are fairly new to AR15.com, welcome. Hopefully you've been reading a lot. I spent about a month reading posts before I bought anything, I should have waited longer.
There are many companies that make what you are looking for. Check the Industry page for links.
As for the build versus buy debate, I bought a stripped lower and a kit and put mine together. As I said before, I should have read more, I would have gone a different route with the kit. If you do decide to build one, you should be able to save much more than $100. If you like tinkering with guns, putting a kit together is a lot of fun and you learn about the workings of the rifle, at least the lower.
From what I've seen and heard you can't go wrong buying a Bushmaster. It's all personal preference.
Good luck and keep us informed.
Scott
Thought I should add a couple things. Rock River Arms isn't listed on the industry page. They have a reputation for making great rifles. Also, if you build, read everything you can about the different kit vendors before you buy. There are many posts about the various companies, J&T, Model 1, M&A...