CA are good. classic army make some very nice MP5's aswell. good choice on that front.
SD5 has a solid stock, doesnt move BUT houses a larger battery (more later)
SD6 has a collapsible stock, doesnt house any battery, it is in the fore grip and room is more limited
yes it is true, large type and mini type are essentially the same other than a large type has a larger, more efficient connection to the gun. also, the larger the battery, the longer it lasts (usually)
upgradeability, no difference, but if you get a new stock you will loose the EPIC battery space
performance, bigger battery can fire more rounds before being replaced. OR a bigger one will fire faster and strip your gears and waste ammo, don't bother with this.
mags, basically any high cap will fit.
sights are easy, you need an adapter with a rail on it, they cost around £15 new but they perform the same if you get them for £3 off a forum (keep an eye out for bargains)
no noobish qusetions, if you want to know something, don't be afraid to ask, it's better for us to answer quickly and truthfully than have noobs going on and on at us :lol:
also, post number 1 - thus:
Welcome to the forums