I have no experience with Bud's Guns. In regards to the DPMS AP4 22 LR upper the bolt is patterned after the Ruger 10/22 and is also used in an upper sold by JP. I believe the bolt/barrel used in the DPMS is a Nordic design and may even be produced by Nordic. One post I read said DPMS did not have parts for their 22 LR uppers but I just checked and they still list two 22 LR uppers for sale on their web site store. The AP4 22 LR upper is priced at $425 so sounds like Bud's has a good price. The DPMS uses a proprietary upper barrel/bolt but since this is a complete upper the only down side is you may not be able to reuse the parts if the barrel/bolt wear out. The magazines are Black Dog Machine and are compatiable with uppers made by CMMG, TACCOM, JP, Tactical Solutions, chiappa and conversion kits like Ciener, Target Master or CMMG.
Shadowcop did a review on this upper a few years ago and another poster did a review onthe 16" bull barrel upper in 22 LR. Here is a link to the review on the 16" bull barrel.
http://www.ar15.com/forums/t_3_15/678545_DPMS_and_Chiappa_22_LR_AR_15_Dedicated_Upper_Comparison.html