I have several different AR lower builds and have found that all of them are within specs. If you are just building one for a fun gun then go with the lowest price. I even have 5 PlumCrazy Polymer lowers with Polymer trigger groups and they work great. You can pick up a complete Lower with 6 position M4 Collapsible Stock for around $150. Anytime you buy a completely assembled lower you will pay more for it. Build it yourself and save some money. Since the 22 conversion goes in the upper any lower will work. Just remember 22 conversion kits are finicky and most of the time you have to tweak it anyway.
USAF Retired, CATM, SC CWP, NH NR CWP, NRA Benefactor
To preserve liberty, it is essential that the whole body of people always possess arms, and be taught alike, especially when young, how to use them... -- Richard Henry Lee, 1787