A quality lower is getting the right stock and grip for you and I would just flat out spend the money and get a Geissele trigger. They are expensive but they are also the best. Hands down!
So I recommend you build your lower yourself. Easy to do it is cheaper than buying and you make it the way you want as far as color and comfort. And you will learn about your rifle.
Your upper will depend on the caliber you choose. I highly recommend either 6.5 grendel or 6.8SPC.
The 6.5G has a flatter trajectory and is a better caliber if shooting 300 + yards. The 6.8SPC has the same ballistics as the 6.5G up to about 250 to 300 but falls short after that. I personally chose the 6.8 because the ammo is more available and I do not reload and I doubt I will ever shoot anything over 250 yards.
I would not recommend building your own upper. If you want quality and you want it done right get with Eric at Blackstone arms. For $700-$900 he will build you the upper you want and it will be a shooter like mine is and about a hundred other guys on this site.
That will bring you in around $1300-1500 give or take and you will have a very high quality AR set up for piggin.
3 most important parts to an AR. 1. Trigger 2. Barrel 3. Optic
That leaves you $1500 for a thermal and monocular and bullets.
Good luck!