Be aware that some venues have an official photographer and can have restrictions on fan photography; and typically if they see you carrying a "big camera and lens" they might well not allow you to take it in (partly on the assumption that anyone with a camera and lens that big is a pro going to work not a fan). They might also not allow entry for bigger gear on grounds of health and safety and also property security (everyone's dancing, someone spills a drink on your expensive lens or knocks it out of your hands).
For music concerts a few thoughts
1) The lighting is BAD. No ifs or buts, from photography points of view the lighting will be bad. In fact I'd say its one of the most challenging situations to shoot with the light effects.
2) You ideally want a lens with a wide aperture (small f number) to help gather as much light as possible. This helps with your exposures AND your auto focusing system.
3) f2.8 or wider (smaller f number) is likely going to be what you're after. If you're a long distance away then you're limited to f2.8; if you're closer options broaden out as you gain the options like 50mm f1.4, 85mm lenses etc... Which offer you a much wider aperture; but you'll either have to have wider shots or be a lot closer for those to work (eg if you want to isolate the performer on stage).
4) The crop cameras perform well and high end (L) lenses work great on them. Indeed many of the wide aperture lenses you're going to need are going to be in the top end of the market. This means a higher price tag a well.
5) Fullframe would be the favoured choice since it gives you a wider angle of view compared to crop sensor, which is of benefit when working indoors and closer to subjects. Fullframe generally also have superior high ISO noise performance. The 70D is no slouch, but fullframe options from the same generation of cameras or newer, will have improved high ISO performance. That means being able to shoot shots clearer and with less light.
Budget - is important. Basically this is one of those situations where both camera and lens are tested at their light gathering limits. This makes it a challenge, but also tends to put a lot more pressure on having good gear (esp if you're not using a flash). A rough idea of your budget would help a lot in guiding you; as well as your overall intention (hobby/fan/aiming to go pro), just to get an idea of where you want to take this interest.
Edit - two lenses one camera is great; but in the practical world you'll find that you won't lens change all that often; esp if you're in the middle of the crowd. In that situation most photographers will have two cameras and two lenses so that they can just let one hang and use the other. A much faster way to change and safer too as you're not fiddling with pulling lenses on and off.
