I can vouch for the EF 70-200 f/4 IS; it's a wonderful lens and a great length on a crop sensor. However, you may want more length, and you may want to spend less. It ain't cheap.
Looking over my images, I think my 2 stop comment comparing the 50D and 5D might be off, but I think the point stands. All cameras achieve high ISOs by amplifying the signal from the sensor, and even with high end cameras, that takes its toll if you push it too high. I did a bunch of indoor flash photos soon after I got my 5D3. The ISO 800 shots were very clean, but the ISO 1600 shots showed some problems. I think the best, unless you really need high ISOs, is to try to get more light.