It always used to be that Buildcraft was a good pairing as it provided the transportation that IC2 lacked without adding much more around it like EnderIO for example would. Thermal Dynamics would probably also work for pipes if you'd rather have a slightly easier experience, albeit less thematically with IC2.
Storage is more of a personal preference, Iron Chests is the classic solution but there's so many options depending on whether you want denser storage (from something like AE2 or Refined Storage) or more expansive storage (such as Iron Chests or Storage Drawers).
Other things you might want also depend on the kind of play style you want. IC2 lacks any wood farming, so Forestry can be useful for that. But if you're not building much to use the wood you probably wouldn't need it. Longer distance transport could be covered by something like Railcraft or Ender Chests as IC2 only offers teleporting players about with teleporters. JEI for crafting and machine recipes is a sensible choice, but that's normally a given for modded worlds.
Otherwise you don't really need much else (as after all you can play just IC2 without anything else), you can always add more mods as you go along if you feel something is missing (or offer a suggestion for IC2 
)
I’ve felt from pretty early on that some sort of wood auto-farming was important for charcoal and energy, not building. But then, I’ve only used IC2 with Gregtech, and vanilla IC2’s energy demands may not be as high.
Forestry’s worth having just for the project workbench. It’s a huge improvement for crafting that’s the core of any Minecraft experience. It’s own attached inventory, a limited memory of recent recipes, and no need to worry about leaving stuff on the crafting grid.
As an alternative to Forestry, I’ve played a bit with Ancient Warfare for auto-farming food and wood. It’s considerably easier than Forestry tree multi farms, and more thematic if you hire NPC lumberjacks to run the Ancient Warfare tree farms. The main problem is the NPC hirelings are idiots who tend to get stuck on trivial obstacles, and get killed if you miss a dark spot and a zombie spawns inside your protected area.
Project Red for redstone logic seems valuable with just about any mod. Redstone wires work SO much better than vanilla’s redstone dust trails.
Railcraft gives you some pretty good tanks in addition to the trains.
I don’t like Thermal Dynamics and its progression, which feels forced and arbitrary. Yet I’m using it anyway for the Mechanist’s Workbench and, to a lesser degree, for its auto-crafter. Signalum Workbenches are expensive since they eat ender pearls, a very limited resource barring an End farm, but they’re like a (mostly) better version of Forestry’s project bench.
I say “mostly” because the top-tier version has a larger inventory and more memory slots than the Forestry workbench, but it’s significantly clumsier to use. The memory slots are manual, not automatic, and there’s no way to craft a full stack of things at once. Still, there comes a time when less inventory juggling is worth the drawbacks.
