if your getting an Mp5 don't worry about a sidearm unless you want one lol, and barrel length has absolutely almost nothing to do with accuracy, my G&G mp5A5 came stock with a reinforced (Steel not zinc) mechbox, and tightbore barrel, shooting 350 at 18 bps. I only play outdoors and it performs way better than my AK which shoots 410fps at 11bps, with a madbull 6.03 tightbore and guarder improved hopup. If your going the Mp5 way I highly suggest this model, there may be better companies out there but for the price and performance this one is crazy. Side note though, if your playing outdoors always use 0.25g plus, they may travel slower than 0.2g bb's when they exit the barrel, but at 90 feet they are traveling faster than that 0.2g.
DMR - Designated Marksman Rifle : basically a sniper, but with semi auto, (in airsoft sometimes they got full auto lol) an m14 would be an example.