[Schwag]

Styrak has a point, he just wasn't nice about it but who cares?

For an mp5, avoid the sd series for a bunch of reasons.
The front end is too small for anything other than a small battery. If you go full stock for a bigger battery, you defeat the purpose of getting an mp5, which is the super compact size.
Also, the sd silencer can break easily and is hard to replace.
If you want to mount a ris, you'll have to change more parts.

The best mp5 to start with is a metal body mp5 ras. JG makes a great one but it could use $100 in parts to make it awesome. Cheap, solid, powerful, small.
It has a ris so you can put on a battery box and keep it small with a great rof.

I took my jg mp5 ras and built it up for a high-power lipo setup and it's MEAN.
390 fps, 20 bbs per sec. Only a 2fps variance.
Because of the solid design of the mp5, they are remarkably accurate at range.