There is soooo much that needs to be considered here that you really should look for a Cisco partner to support you. But in general: The MX is typically "only" the firewall and doesn't care if the network has APs from vendor A or vendor B. With Meraki APs, the MX can take the role that is known as an anchor controller in the traditional Cisco wireless world. It is not possible to mix traditional Cisco and Meraki for this function. If you buy Catalyst 9100 APs, you need a compatible controller which could be any of the c9800-series. Your older Aironet 1832 can still run on this controller.
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