The M6 is a very powerfull light, maybe overkill for 100-200 yards. Surefire does make a weapons light as powerful as the M6, put it is very bulky and heavy. Whicked bright though. Its the M500B. It will replace the handguards on your rifle, and it will also make mounting other accessories, like a bi-pod, more difficult.
A better option might be the M981 light Grant suggested. Or even the M971 light. You will need a rail of some sort wo mount this light. You mentioned wanting a mount to use on a rail, do you have a rail already? Anyway, here's the M971 and M981 lights. In my opinion, these would be sufficient for 100-200 yards. Especially the M981. I have one of those and it works very well out to those ranges and further.
M971xx
M981xx
If you have a rail of some sort, you can get pannels to cover the rails that hold the pressure pad. Tango Down has the one most are familiar with. But Magpul just started to sell one made by Falcon Industries as well. You can also get a couple forward grips that have cut outs to hold the pad. Tango Down and Dieter CQD make them.