Welcome from another FL guy!!! Where in FL are you? Here's some opinions on your questions...
1) You can Build your own, but please get a kit from a quality source like Bushmaster. Or just buy a complete lower from your FFL and then order a complete upper from a quality company. Buying it from a shop, or here online would be easier and quicker in most cases. I dont know exactly how much you'd save doing it yourself.
2) Generally, no, a Colt will shoot just the same as any other quality brand.
3) Chrome lined barrel will be a big advantage to almost all shooters out there and getting one is a wise idea unless you are building a super accurate, tight tollerance bench gun. Unless you are doing that, go chrome or go home.
4) Warranties are only as good as the company who offers it. Some companies offer a lifetime warranty but will weasel out of it any way they can. Others offer a 1 year warranty on paper, but in practice are well known to take care of their customers for as long as they own the rifle.
I generally say stick with the ABC's of manufacturers (Armalite, Bushmaster, Colt) and maybe a Rock River with a chrome lined barrel. Keep looking around and you'll see a lot of debates surrounding the small but somewhat interesting differences between the manufacturers.