I think I'll buy 9105AXI. Is there some resources to study controllers? Like compatibilities, firmware, List of outdated controllers etc.I've read documentations on cisco website but it's very confusing.Thanks

Or can you just explain how to choose a correct/supported one?I read the link you posted but it's not clear to me. Thanks

I was looking for a cheap controller. Can you help me? 

Is AIR-CT5508-12-K9 compatible with c9105axw?

Or any inexpensive controller? My client need to have just that 2 APs