The UVA/UVB in 1 bulbs like PowerSun (Mercury vapors) etc can't be sitting on any kind of mesh, it'll burn right through it.
Mercury vapors are a bit overkill for chameleons, although perfect for bearded dragons and desert species.
For a basking light, the wattage totally depends on your specific setup. How close he can get to it, your ambient temperatures etc. You may have to try a few wattages to find the right one. I go with regular house bulbs from home depot (The clear ones, not the white, just a preference).
For UVB I would recommend a 5.0 ReptiSun tube (comes in 12", 18" or 24")