See the pistol section. Unless there are state laws to mess with you do not need a special pistol lower. Any never assembled into a rifle lower, stripped or complete (so long as it has never had a stock attached) is all you need for a pistol build. Just be sure when you do the 4473 the word "rifle" is not used in the description.
This special pistol lower business is just a way for sellers of lowers to make a few extra bucks on the unsuspecting buyer UNLESS there are state laws that must be satisfied. I know the federal law but cannot keep track of all the stupid state laws so you will have to check that out on your own.
See my letter from the ATFE in the AR Pistol board.
Crap, misread your question. I can highly recommend CMMG. They are great people and build a very nice product. Since you want a POF upper I guess you would have to get it from them. Any length barrel can be used on a pistol build but it is best to stay less than 16 inches, just to cut down on the confusion because with a 16 inch barrel and entry buffer tube OAL is over 26 inches and except for the lack of a stock that meets the definition of a rifle. If you put a 14 inch POF upper (if they will make you one) on the pistol lower you will be fine.
Again, some states may limit the length of a pistol barrel but there are no federal restrictions.