as far as i know you don’t need to care about the credential handling in your check. The function snmp_scan_function internally handles all the information, if it’s version 1, 2c or 3, and just gives you back all information from the oids. Just check other SNMP checks within the folder ~site/share/check_mk/checks who to deal with.
Also take a look on the documentation and howto:
https://checkmk.com/cms_devel_check_plugins.html
https://checkmk.com/cms_legacy_devel_snmpbased.html
EDIT:
The info variable in your check and inventory function should already contain all informations you need.