…unfortunately MT has no redundancy concept for capsman build into RouterOS (AFAIK).
In a smaller site, typically there will be one MT device, that will be the one critical element if it fails…most likely a Router or Switch.
I’d rather focus on PSU redundancy and leave capsman on that single device, instead of having a hot spare capsman device (where you would need to frequently sync the config changes accoss),
You can/should cater for a cold spare of this device and then import config backups, as usual.
A RB4011 and Hex-X allows for POE-In and an external PSU, so this is also possible here.
With local forwarding set to ON in datapath, performance wise any device will do, in my opinion to cater for number of CAPs, as you mentioned.
Alternatively, see here: http://forum.mikrotik.com/t/how-to-make-a-capsman-redundant/131781/3