If you're on a budget, then you need to think about completing the lower yourself. It's the best way to start, it's simple and easy to assemble.
Spike's Tactical makes some of the most popular lowers, if you can find one? They are that popular!
They're all machined from forged 7075 aluminum and finished with a hard anodizing coating. Spike's offers different model lowers that are all about the same with the only differences being cosmetic.
There are other manufacturers that offer similar packages and are more affordable. Stripped lowers can cost as little as $75, and often are found on sale for less.
CMMG, DPMS, S&W, SAA, and Aero Precision, to name a few. I use CMMG for budget builds and have never experienced a problem with them. I also like SAA and Aero Precision products. They're top quality for a budget price!
The best part is learning how all the parts are assembled, and learning what makes your rifle work. Then when something goes wrong you'll have an idea where to start.