Search & Destroy is excellent but you need lots and lots of practice to even get a kill in it. As for guns, the Thompson is good, it's unlocked right at the start, has little recoil and with Round Drum it's got lots of bullets. You can never go wrong with the Type 100, MP40, Machine Guns or the STG-44. I love the MP40.
There you go, lots of options at different levels. I wouldn't recommend ranking up as it doesn't take any skill at all and you'll have a tough time prestiging or even playing a match at all after you reach level 65 if you don't plan on doing prestige. Just do it the fair way until you think you're good and do some rank ups occasionally if you like.