Thermal Expansion is purpose-built for maximum performance. BC has been improved upon a LOT, and unless you have a shitton of resources flying through their transparent pipes (which causes framerate drops), you should be ok. Ars Magica 2 adds a couple of worldgen structures, but none of it's blocks is particularly resource hungry. Thaumcraft 4 is one of the most well-made mods ever. It's perfectly balanced and it also doesn't have much of an impact in resource consumption, except in some very specific parts which consume slightly more resources, like when you're infusion crafting.
Applied Energistics is an almost lag-free and very cool storage option. Useful alternatives include of course the Iron Chests mod, and i tend to prefer the JABBA barrels mod instead of Factorization Barrels, because i rarely use anything from Factorization apart from barrels, wrath lamps, and, sometimes in very specific builds, routers. Logistics Pipes is now also an option, and it can have nice interactions with Applied Energistics, but i'd advise against relying solely on Logistics Pipes for your sorting because it does tend to increase the amount of resource consumption.
IC2 is a nearly dead tech mod, since nearly everything you can do there, you can do via BC/TE. Quantum Armor might be missed, as well as the Jetpack, but both can be completely replaced by Modular Powersuits, which is better in nearly every respect in regards to Quantum Armor. Don't bother installing IC2 unless you also install Gregtech, because only then IC2 gains real value. A relatively new tech mod that has been released and is interesting is RotaryCraft - downside is it has it's own power system that works out of RL principles of energy provided in terms of torque and speed applied to gears or gearboxes, which makes it rather difficult to master. It's approach is much different from the other tech mods, as it strives to provide realistic energy generation scheme and transfering of power. Upside is that it offers one of the best ore-to-final product rates. Above 5x yield for some ores. But it is SLOW unless you provide the machines with a crapton of power via torque/speed increase. Mekanism is an interesting tech mod, and different enough that in general it is worth a shot - it also provides up to 5x Ore-to-Final-Product ratio, and it IS compatible with TE/BC power - But also has it's own power (Joules, different from MJ - Minecraft Joules), since it's part of the universal electricity API. But you won't even notice that since the machine will happily accept whatever power you provide it with and work just fine - be it RF or MJ energy. What it also does, is bring it's own jetpacks out into the table - one more nail in the IC2 coffin. It's quarry is also the most cool looking one, just saying. Digital Miner is a beast.
A server shouldn't need much power to handle these mods i outlined, especially if it's only for a few people - max of 10~12 simultaneously online.
In servers, what you need to look out for, is tick-rate lag (caused most often by having a crapton of tile entities updates, like machine animations, items flowing in pipes, light updates, redstone updates, often in chunloaded areas), and framerate lag, which is caused by rendering animations and graphics, and is normally clientside. As long as the server has a high uptime, server lag won't be an issue.
Another good tip if you're concerned about resource consumption: ExtraUtilities offer better solutions to pumping lava and quarrying - the Enderthermic Pump and the Ender Quarry. The first, when pumping lava source blocks out, will replace them with smoothstone therefore minimizing the amount of flowing lava blocks and the rendering created by them to induce lag. The second, replaces any mined blocks with dirt, so you don't completely destroy the terrain and preserve natural caves, etc. It's pipes and sorting system are also pretty good and there's just tons of useful blocks, like the Angel Block and the sound mufflers. If you haven't heard of it, check it out. Once you have a modlist let me know and i can maybe make a few suggestions.