Good day guys!
I would like to ask if a Mobility Service Engine is really needed to make Clean Air functional on a Cisco 3500?
No. All you need is 7.X to run on the WLC.
One thing though ... For CleanAir to function optimally, it's recommended that you use 3500i/e exclusively. You can't (you can but CleanAir won't run clean, excuse the pun) mix the 3500 with other non-CleanAir APs.