The good with el-mod is that the pre-set profiles and the ability to supply hig power allows you to choose anything about the inner mechanics.

I have a King Tiger with 3:1 and servo recoil and elevation, and a Sherman with stock platic gears and HL recoil (well kind of, I converted an airsoft unit to recoil).

Both have plastic (stock) hull and wheels, just metal for sprocket and drive.

Both run great, both are a pleasure to drive.

My personal answers therefore are as follows:

1) Yes, but needed only if you plan to build a tank equipped with heavy metal parts, such as metal tracks or hull. Otherwise is not needed.

2) Servo recoil and elevation is better, more precise, less space occupied and less troubled mechanics, but you need the PRO unit to support that.

3) Metal hulls are nice, but heavy. That means less battery duration.
I prefer plastic hulls with metal stiffners for longer playing time, but heavy tanks also add realism to drive (but the mechanics have to be tuned for the increased stress).

4) Nope

5) Elmod forever, is the way to go for the experienced tanker who enjoys pure driving in addition to battling.