Monks do not need to meet prerequisites for their feats, but do not lose them when wearing armor - they only lose speed bonus, AC bonus, and flurry of blows.
Stunning Fist has a save DC of 10 + 1/2 HD + Wis modifier, and you can use it once per every 4 levels, no more than 1/round. (In the Special section, it says that you get 1 use per monk level rather than per every 4.) There's also clarification of what can't be stunned - constructs, oozes, plants, undead, incorporeal, and creatures immune to critical hits.
With Improved Grapple, you do not provoke an AoO when makign the touch attack to start a grapple. You also gain a +4 on all grapple checks, regardless of whether you started the grapple.
EDIT: Forgot to answer the evasion question - monks get it at level 2, and improved evasion at level 9.
J