i say if you can get one, get one set up with a flat top, that will give you more options, a 16" barrel an optic ie eotech, buis, sling, i prefer the one points, good mags, ammo and a couple training courses under your belt too.
look at stag, bushmaster, slr -15 of you can afford them and you will be good to go. caliber wise i say stick with what is avaliable most of the time, well more so than the other odd calibers. get the .223/5.5.6