Thanks Ghost Dog and Colohunter.
The Brownell's link clarified the buffer parts question right up, and on rechecking the DPMS lower parts kit on Brownells I do see the buffer retainer and spring listed. I have priced out the five individual parts at Midway and I came up just short of $38 - all the parts are DPMS except a UTG buffer tube for $17, which is at the very low end of the price range. The reviews on it are uniformly favorable, but there are only two of them. The DPMS kit at Brownells with the DPMS buffer tube is $99, which implies the buffer tube is over $60. On Brownells, DS Arms has one for $25, but the notes indicate some finishing might be needed to get the best fit; VLTor and Double Star are between that and the most expensive, which is High Standard at $55. I don't want to cheap-shot this build, since it will be for my 450. Is the UTG a good deal or too good to be true and should I be looking for a higher priced spread (and which brand)?
Joe Bob's kit looks good, but it has several components trigger group, hand grip) I plan to replace with personal preference items and it is pretty pricey. (OK, you get what you pay for.) The DPMS kit at Brownells does not include the trigger group (but does include the springs, for some reason) or the hand grip, so it is a better fit (and at $36).
Many, many thanks for taking the time to reply. Please feel free to let me know any other info you think might be of use, including your personal preferences on manufacturers.