/testfor levels in Minecraft
You can test whether a player has a Diamond by running the following command block, with a comparator coming out of it.
/clear @p Diamond 0
Your comparator will be the output of side one of your AND gate.
You can test for a player's level by using the selector modifiers:
/testfor @p[lm=10]
As usual, a comparator will have to come out of this command block. Its output will be the other side of your AND gate.
From there, you can create an AND gate as per normal.
To test for the conditions, simply run both command blocks at the same time.