It's well know that I prefer the AR platform to any other battle rifle. The reasons are due to it's proliferation in the US as well as my familiarity with the rifle series. It's the most common rifle sold in the US for a long time now and it really is well suited to 90% of shooters due to it's modular and adjustable nature. You can build them in literally any configuration imaginable.
Another significant belief I have was first iterated by good old Joe Stalin. "Quantity has a quality all it's own." So for the price of a high end manufacturer's AR15, I can build 3 budget guns. Once again, why? Because even a budget build can work remarkably well. Outfitting another person can be more of a force multiplier than a super whiz bang piston driven death dealer.
These builds tend to run around the 650 dollar mark and include a 1x optic. Why an optic? Because it's a crutch. Teaching fundamentals with irons take time, and it's much easier and faster to teach fundamental hold and bullet drop.You might not be outfitting a shooter, you might be outfitting a neighbor.
What to look for in a budget rifle:
Standard quality parts
Mil spec dimension parts
For this discussion, there are some built rifles that accomplish these tasks. Smith and Wesson's base models as well as DPMS's base models work just fine. I haven't had much luck with PSA or some of the other cheapo parts like UTG and the like. Double star on the other hand have been nothing short of brilliant in my rifles. Many contractors are given DPMS rifles for ship defense and foreign protection details. They work and they're cheap. I've been using the DPMS oracle uppers for all the builds I've done in the past 4 years with a perfect track record. No functionality issues, decent accuracy and a cost around 300 bucks. Hard to beat with an included BCG and charging handle.
Optics are an easy choice. Something inexpensive and durable. Vortex is a good option for this. The Sparc series is pretty good, and there are a myriad of others out there that will work great. Always have extra batteries on hand. I include flip up iron sights as well. I've been prone to getting a gas block with a picatinny rail and dual flip ups die to wanting a clear sight picture. A standard front sight post occludes your sight picture.
I generally install a JP reduced power spring kit to loosen up the trigger a bit. A cheapo sling works great, and it is easy because the rifles I build are very light weight.