ISE Base license will handle Device Administration over RADIUS.
ISE Device Administration license will handle Device Administration over TACACS+.
All of those vendors support TACACS+, so if you want to see all device administration in one place and manage it the same way use TACACS+ unless a device does not support TACACS+
If you are not using ISE for network access or other features besides Device Administration, your can get the smallest base license possible (L-ISE-BSE-100=), or if your customer has C1 licensing on their devices they may already own Base licenses.
The Device Admin license is also perpetual license (L-ISE-TACACS=)
Once they see ISE, they will probably be interested in looking at more network access functionality, but this will get you started.