This post by Thomas provides some more info for POC purposes, it's been updated for 2.6 but the same applies to 2.4. Without a valid service contract/purchasing of ISE, you won't have the rights to download the other OVA's, just the ISO and eval. If you want to do a POC with more than 100 endpoints then you can buy some extra licensing, work within the endpoint licensing enforcement grace built in to the product(it's not an immediate not a hard stop at 100), or email licensing@cisco.com and see if they will provide a higher count evaluation license.
https://community.cisco.com/t5/security-documents/how-to-get-ise-evaluation-software-amp-licenses/ta-p/3630795As for what you should download, grab the evaluation OVA, ISE-2.4.0.357-virtual-Eval.ova. It says 100 endpoints, but that's because if does not have resource reservations like the others, it's only given 2 cpu cores. You can assign more cpu and it will run the same as a 3515. While you could build and deploy an ISE node from the ISO, I would recommend the OVA since it will include the VM template that isn't well documented online.
If you think you need some help with a POC, it would be beneficial to contact your Cisco partner, there are specific POC resources that we as partners are able to access to ensure your pilot is successful.
