Pretty much everything else depends on you and what you want to get out of the class. If you're going to be on objectives, Flak/Tac. If you want to play as aggressive as possible, Dexterity. If you plan on staying alive for a long time, Scavenger.
For attachments, Quickdraw is great for slower firing weapons such as the MSMC since you can get bullets onto target faster. If you have great aim, Quickdraw is a great tool. Otherwise, you might find Laser more effective for more freedom in strafing. I would recommend one or the other as your first attachment, though bear in mind that the MSMC and PDW are the worst subs from the hip.
The 2nd attachment is up to you. Long Barrel gives you a better 3 and 4 hit kill range. Fast Mags I don't really encourage since the MSMC reloads quickly by default. Extended Mags is a good substitute for Scavenger + giving you a larger mag. You could go Quickdraw AND Laser if you'd like, I do that with the MP7.
My current MSMC class is Lightweight, Flak, Scav, Toughness, Tac, Quickdraw and MMS.