Welcome!

Many opinions here and most are worth exactly what you pay for them. I normally suggest you build your lower. You get the benefit of knowing how to maintain and swap out parts on the fire control group. You also get exactly what you choose and you can easily make changes later down the road. For a first upper, my preference is Bravo Company and I like mid-length gas on a 16" barrel. Twist rate has more to do with the bullets you plan on shooting. 1 in 8 is a good compromise and you will be able to shoot almost anything with that twist rate.

Remember to budget for your sights/optics, magazines and ammunition. Over the life of your rifle, you will spend more money on ammunition than any other part of the rifle. At least with good optics you can reuse the glass on something else.

Decide how you will be using the rifle and then look for suggestions here as well as all over the Internet.

Be safe.