There appears to be no benefit. While your character's level and skill selection transfers over, I don't seem to have retained any of my equipment or upgrades.
Normandy upgrades are retained, but those have other impacts across transferred saves as well (Suicide mission survival), so it's unlikely that you won't bring those over unless you actively avoided them.
So, the way combos in ME 2 work is that they have two components, a primer, and a detonator.
Whenever a target currently affected by a primer is hit by an eligible detonator, there's a chance that explosion occurs based on the specific combination of effects in play. For example, Biotic Charge is capable of detonating all four varieties of combos. If a target is hit by Warp first, which acts as a primer, a Biotic Combo will trigger, which explode in an area effect for decent damage against health, barriers and armor. Biotic Combos are also notable in that they are the only Combo that triggers 100% reliably, regardless of the health of the target. Others may require a killing blow, or a fair amount of luck.
If instead of charging a target affected by warp, we had charged a target that had been set on fire by Incendiary Ammo, we'd have a chance of triggering a Flame Explosion, which will set victims on fire, causing organics to panic.
There's a whole variety of combinations that can produce one of four types of combos (Biotic, Flame, Electric, Cryo), so experiment!
With regard to the various Detonation upgrades, they specifically effect combos detonated by that power. As a result, they are incapable of stacking.
Best Answer
Weapon mods become available for purchase as you complete missions. Some or all of them may be actually available to pick up for free in the mission they appear after, in which case you won't have to buy them.
It is entirely possible to have all weapon mods at level 5 by the end of the first playthrough. Some or all of them may be downgraded a level, presumably to populate the citadel stores.