No problem putting a 9mm upper on a regular AR15 lower. The 9mm uppers as a general rule will fit and work on any AR15 lower. Sometimes you need to do a little filing on the upper to get it to fit with certain lowers, depending on what upper and what lower you use. If you are using a post-ban lower, then you have to make sure that the upper conforms to the assault weapon ban regs. What kind of adapter and what mags you will need depends on which upper configuration you use. If you go with the Oly Arms upper, you can use individually modified Sten mags or you can buy one of the SOCOM mag blocks and use Sten mags. If you are going to use one of the uppers that is based on the Colt configuration, then you will be limited to using Colt factory mags or Uzi modified mags. With the Colt configuration, there are several different manufacturers of adapters you can choose from.
Charles Tatum
Alamo Professional Arms