Some folks like to use Quickdraw on it but if you plan your engagements out well enough you won't need it. Personally, I like using Amplify with it just in case someone does get closer to me. It allows me to be better prepared for the fight. I also like Blast Shield just because smart people are going to try and nade you. Depending on how much you move around you might to think about adding double tacticals with the motion sensor. They can come in quite handy when moving into tight areas such as central command on Overwatch. I always liked to throw down a motion sensor behind me in key areas as I moved through to clear the building. They tend to slow down those shotgunners and the sensor of course lights them up so that you don't have to guess where they are. Generally I didn't bother with Sleight of Hand for my Marksman classes but I did either carry a pistol or picked up a gun off of the ground when I got a chance to cover those moments when I would get surprised while reloading.
Personally, I like the M14. Here's my current setup:
M14 - Muzzle Break, Thermal Scope
Pistol of choice (Currently the MP-443 with Muzzle Break)
Ready Up, Marathon, Quickdraw, Dead Silence, Focus
Strike Package (Support)
Ballistic Vests, Squadmate, Oracle
The MK14 is my favorite marksmen, but I would say the MR-28 is easier to pick up if you aren't used to single-shot weapons. Since the guy above gave you a good MK14 class, I'll give you a good MR-28 setup.:
MR-28 - Red dot sight, armor-piercing rounds
You can swap out armor piercing for silencer, but I would rather have the ability to shoot players behind cover. Also, the muzzle break isn't really necessary because it's only going to make one bullet difference, and the MR-28 has a fast fire rate. The default scope is kind of distracting, and I feel red dot works the best at range. On smaller maps Iron sight is the way to go because it's clean and grants you extra mobility which helps in mid-close range gunfights.
Ready Up, Sleight of Hand, Marathon, Dead Silence, Focus, Blast Shield
Thanks guys, I will be trying these out and seeing what works best for me. Any other tips/hints for a noob marksman eventually moving up to sniping proper. I've played this series since W@W and have never liked anything with a scope except for my LMG.