Be aware that AR parts are becoming scarce at this time. Folks are scared that Obama will be re-elected and outlaw guns the way Clinton did in the 1990s.
For Grendel and Beowulf, look for the hard-to-find parts first. That would be barrels, bolts and magazines. With the barrels in hand, you'll be able to examine the barrel extensions and determine if you need M4 cut or non-M4 cut upper receivers.
You'll probably be able to find all the other parts but prices will be high compared to what they were a year ago. Order parts from those companies who tell you they have parts in stock. If the website doesn't tell you about their stock level, assume they are out and go elsewhere (or be prepared to wait for months).