For best weapon, go for the MP5: mid-long range (THE BEST weapon overall, even if stock can take out sniper 100+ meters on full auto with 10-15 bullets).
Best weapon for CQC is MP7 without doubt: close-mid range
Combine the two (buy dual weapon slot), both with stab stocks, casket mags, precision barrels and mp5 with m145 scope and mp7 with rapid-aim and you'll have THE ULTIMATE engineer set.