I'd go for the AK-74. I know many will disagree but both the M4 and K2 are so common and mainstream (but both are still awesome guns). I started with the 74 and I love it. Certainly helped me get early training for the AK-47 and KTR-08.
I would suggest having Decoy Grenades for use in DM. When you throw that grenade, it will create firing noises of any weapon. It's kinda helpful to buy your team some time while you're planting the C4 (Black List Side).
Hmmm to divert attention? This will work on GR side too like ninja defuse or clutching