You do not want any of the legacy controllers (200, 800, 2400, SC-1, or SC-2) as they can only run up to AOS 5.x. We are on AOS 6.3.1.3 GA and 6.4 BETA. You would be missing out on a ton of cool new features.
The 600 series is end of sales. While they can run current SW they do not get all the new features as they are resource constrained.
That leaves the 3000 series or the newer 7000 series. If you get a 3200 make sure it is a 3200XM (extra memory) so it can run current SW. If you do not get the XM version you can purchase the SW upgrade from Aruba. Even the 3000 series will be missing out of some of the cool new features of AOS 6.4 like Deep Packet Inspection.
So, for the purpose of preparing for the current ACMA and ACMP I would say a 3200XM is the minimum and will work well for some time to come. If you want to ensure you have all the latest and greatest features you will need to step up to the 7000 series. We do have a couple of small 7000 controllers coming out soon.