It wasn't all that long ago that I was a newbie to AR's myself, so I had a lot of the same questions you do. Here's my two cents worth. Just some things to consider.
Don't be so anxious, or so thrifty, that you get a system you may regret a little later on. Instead of making compromises to save a hundred bucks or two, take cold showers and keep feeding the piggybank a few more weeks or months.
I wouldn't suggest building from a kit as a newbie. Maybe later on, if that appeals to you.
I won't knock any particular brand, since I don't have experience with others, but I have been very happy with my Bushmaster. I've not heard complaints about their customer service, although I have not had any issues to bring to them myself. Chrome lined barrels are standard with Bushies. And Colt, of course, has a good rep, but more cost.
Again, this is me. I would much prefer to get my complete rifle from the same company instead of mixing uppers and lowers, unless there is some combination you can't get any other way. Others couldn't care less about this. That's fine.
16 inch barrel. A good compromise length and a good one to start with. My strong opinion is that if you get a 16, make sure you get a true 16 in. barrel and not a shorter one with the permanent muzzle brake attached to bring it to the legal length. Later, if you decide you want a longer barrel, you can swap out for a couple hundo or even get a complete upper for quick switching back and forth.
Barrel profile. Up to you. Heavy, super light, M4 profile. I have the M4 which some think looks cool, and it's mid-weight compared to the others. If I had it to do over again, I would have gotten a fluted heavy barrel from Bushmaster for an extra 50 bucks. But, that's just me.
Chrome. Up to you. Me, I like the idea of a little more durability and ease of cleaning over a little more inherent accuracy. Your priority may be different.
Buttstock. Your call. I like the flexibility and storability of a collapsable. If you get a long barrel, then it doesn't make any sense to get a collapsable.
Flattop receiver. No question about it--you'll want it for all the great optics options you can go with. You can save some money by not getting the removable carry handle/rear sight and get a good flip up rear instead. You can always get the handle later if you want to.
Good luck and have fun.
K