I'd suggest building your own lower, then buying an assembled upper, BCG and bolt handle. Lowers are easy to put together and buying an assembled upper avoids any issues with the gas system and barrel install. To assemble your lower, you'll need an armorer wrench, punches, soft face hammer, and a takedown pin tool.
What that would look like is....an Aero M4E1 lower, Geissele SSA-E trigger, and Battle Arms Development Ambi Safety. A lower parts kit (no FCG) from Aero that has the threaded screws for the rear takedown spring and bolt catch, or a CMMG/other lower parts kit and get the screws from Aero. A BCM/B5 stock and Magpul Grip of your choice, Geissele super 42 buffer and a quality buffer tube (with castle nut). If you're lefty, there are some cool mag releases that make it nice for a lefty to drop mags with theit left hand.
An upper from Aero would work and I have two myself that would great. If i were to get another, I'd look toward BCM or Geissele for a complete upper or I'll just get a new barrel for my current upper. Get a Geiselle charging handle and a good BCG from Aero or BCM. Nitrided bolts are nice, I wouldn't pay extra for a Nickel boron one unless it was on sale. 16" is a good go-to. Later on i got a 12.5" to make an AR pistol but I'm not a fan of the brace in general. The 12.5 is very handy with a suppressor though
All the parts mentioned above can be found on sale through the manufacturer and Primary Arms regularaly, don't pay full price for anything except the actual assembled upper. You may have to pay full price for the upper you want, if you get set on something
Cool thing about ARs is you can switch out and upgrade almost anything as you learn what you like.