Paco it's about time you got a good avatar up!
The hardest single item to buy is going to be your lower receiver. Deciding on which brand you want. I've owned Colt's, Bushmasters, Olympic Arms, DPMS, and cough choke gag... American Spirit Arms. Make sure the lower is a Forged unit and not just cast. Then be sure the barrel is Chrome lined. There are some vendors selling new M4 profile barrels that are not chrome lined. The chrome lining helps with cleaning and corrosion prevention. Like Paco said, you can buy a whole kit minus the lower receiver for around $450 to $500, then just buy the lower receiver... a couple hours of work and you've got an AR15.