Diablo – the earliest act of inferno where you can find the highest item level

You should be using magic missle, with the rune that adds 4 arcane pwer per hit. Then Tornado, with the decreased arcane power rune. Don't forget to use energy armor, with the rune that adds all resistance. And diamond skin, with the rune that adds more damage absorbtion. Also use arcane hydra. And lastly, either magic weapon or archon with teleport and that should do it. Gearwise, get all resistance gear with as many sockets as possible.

For your first question (item/monster levels), I came across a chart someone made to illustrate experience gain, but which can be applied to this as well.

Chart

Source thread on reddit by "Malloy626"

Basically, the cells marked as 100% will be the min, mid and max monster levels in each Act. I doubt all the data is entirely correct, but it gives a good indication on what maximum item levels to expect.