Security Plus license is platform specific and it's only available on ASA5505, 5510, 5512-X.
Here is the model comparison for your reference:
http://www.cisco.com/en/US/products/ps6120/prod_models_comparison.html
Security Plus provides extra feature for those 3 platforms.
VPN Plus is the default ship in 5520.
Yes, you would require separate license for AnyConnect for Mobile and AnyConnect Essentials.
Here is the URL for your reference:
http://www.cisco.com/en/US/docs/security/asa/asa84/license/license_management/license.html#wp1487574
Hope that helps.