M8 is high end imo.
Have a look at Xperia Z1 Compact. It have 20.7 MP Camera and 2MP front. Water proof + dust resistant. 2300 mAh battery with 670 hours standby time.
2 GB RAM + 2.2 Ghz quad core + 4.4 Android kitkat so it won't lag.
I can vouch for the sony experia compact, great phone if you need a small phone that does everything you want. Or if thats too small then the gs3 not bad assuming you don't want to game on it with the highest settings.