I would either buy a 16" or 20". I own a 20" and will buy a 16" someday, but not in a big hurry to. I would get one with a flat top upper, and get the carry handle sight.
Shoot it with the carry handle sight to get the basics down. You will be suprised how well you will be able to shoot with the basic sights.
The reason I say to start with the basic sights is that they make have a consistant sight picture and will show you if your moving your face around on the stock or not.
Far as rate of twist, I would just get the 1/9 twist since its seems to be the most popular should shoot all from 55-62 grain just fine, and maybe even the little heavier stuff okay too.
Far as getting one that is chrome lined in the bore, I would say get it. It sure does make cleaning the rifle alot faster. Supposedly it cuts the accuracy, I can see this. I also am able to shoot around 1.5" @100, on sandbags, with mine if I do my part so think that is good enough for me and mine is chrome lined. That might no be accurate enough for varmits though, but it is with open sights, and cheap russian ammo.
I shoot the cheap russian ammo, sometimes, especially if I am just shooting paper, since the range I shoot at no rapid fire is allowed, so it doesnt heat up and cause problems with the laquer coated cases.
The major brands like DPMS, Rock River, Bushmaster, Armalite, Olympic, should all work well for you.
You dont need to go with anything incredible fancy, I would just go with a standard 20" with a detchable carry handle, chrome lined 1/7, 1/8, 1/9 twist. A2 buttstock. Get a good set of optics for it if you want the magnification, for varmits.
If you fall in love with the rifle you can always buy a 16" upper reciever assembly and have a carbine length as well. I like the Carbines as well, but really like shooting my 20" with the Iron sights.