The first choice you have to make is what kind of magazine - Colt/Uzi or Glock. JP makes a nice lower for Glock mags. For those of you that want to convert in-MA lowers, or prefer to use low cost and readily available UZI mags, the Rock River conversion block is excellent. The RR block is bottom loading, but locks in place by turning a allen bolt that pushes grippers out to the side. This works wonderfully, and does not scratch or mark your lower, and no modifications are required.
Also, you will need a recoil spring/buffer designed for 9mm. I use the JP captive unit (remove the part that holds this in place when the lower is removed if you go with this model, otherwise disassembly is a real pain)
As to UZI mags - you have to cut a new mag release notch, and lower the opening at the back of the magazine - then they work perfectly. The only catch is that you do not have bold hold open. If that causes you problems with only 32 rounds, get the Taylor Freelance UZI mag extension for 42 rounds. Cheap pre-ban Uzi mags are available on line, or in stock at GFA Armsteck, Natick. I had my Uzi mags modified on a $300K milling machine, however, the local USPSA Grandmaster at Hopkinton did his on a dremel and those work perfectly, just not as pretty.