One case that can pretty much do it all in the mid-power range (E-G) is the AeroTech 29/40-120. I have only used mine for G64s but there are a total of 8 different reloads that are compatible with that casing. Biggest downside to this RMS is that the closures can only be used with that casing.
A runner-up to that would be a 29/60-100 system (either AT or Dr Rocket). Big difference there is that you get 2 casings (a 60ns & 100ns) with one set of closures. Biggest advantage to that system is that the closures work with the remainder of the 29mm casings (180, 240, & 300). However, it seems like the reloads for the 60 & 100 casings are harder to find. YMMV...
Only other choice is the Pro38 1 grain. While undoubtedly the easiest of the bunch to use, it is far & away the most expensive at ~$20 per reload. Other downsides include only 2 different propellants available (classic & Smokey Sam) & those reloads are "only" G motors. And Pro38 motors (regardless of size) always ignite soooo easily!
I'd definitely go with the AT 29/40-120. Flies everything from an E16 to a G64 and a ton of motors inbetween. I have 2 of these casings in my collection. It's nice to use the 29mm casing with an E reload in the AT Mustang. I'm not a big fan of motor adapters.
If you're not too sure about reloads, the markings, and how they assemble, definitely start with the 29/40-120. Check your kits and see which motors they can handle. Once you've settled on a motor or reload, you can work through assembling the motor with a friend or by carefully reading through the instructions.
Reloads are great. They add yet another element to this hobby you won't find in a SU motor.
You could also consider getting the 29/120 motor that just came out. Pretty much the same as the 29/60-100 cases and closures, but the 120 has a little more power, and AT is (planning on) making loads for it. Thus, it would be easier to find loads for that motor than the 60 or 100 case (although I love the G104T)