honestly, I've been in your boat.
I have no qualms about used gear. ARs are not complicated to work on. When I was in your position I did this:
Bought a used Calvary arms lower. Yea the company is defunct and if you crack the polymer you might be in trouble, but mine has been completely fine. Also I suspect they will be back in action as a company eventually.
After shipping and transfer it ran me 115 bucks. Make sure you get the pins with the lower.
Buy a LPK, buffer, and spring. total from palmetto will be about 65 bucks. You can sell the a2 grip and recover between 5 and 10 of that, as you won't need it.
At this point- you have a complete lower for about 175.
Alternatively, you can occasionally get stripped lowers from companies like palmetto, DSA, or others for 55-60 shipped, plus transfer. Figure about 75-80 bucks all said and done. PSA has a kit for a complete A2 lower build for about 120. so you could go a traditional lower route for about 200 bucks. I've done it both ways and have little preference. The Cav Arms lower is A1 length, so if you prefer a shorter LOP it is probably a better option.
For uppers, if you are in a no-ban state you can get a DSA for 275+ shipping, then hawk AR15.com for a complete bolt and charging handle (80-90 bucks) and you will have a fully functional rifle for about 530.
If you can be patient, you can find some killer upper deals used on forums. Especially if you don't mind A2 style guns with carry handles- which honestly, I think is better on a budget build as you don't have to buy sights.
If you buy your gear used, and are patient, I think it is very doable for right around 500 bucks- and also you can spread out your purchases to make it a less painful experience.
Another option is to buy a Del-Ton rifle kit for 465, pillage it for the parts needed for the Cav Arms receiver, and sell the left overs to try to keep cost down. Depending on what you can get for the stock, buffer tube, a2 grip, etc- you could probably sneak under that 530 mark previously mentioned and have all new stuff (except the receiver- which occasionally do come up in 'like new' condition).