Chosen Solution
I just got a mid 2011 iMac running OS X El Capitan. If I buy the upgrade kit would I be able to update my operating system?
You should be able to upgrade it (to high Sierra) without the kit.
Here’s a link to the macOS files How to get old versions of macOS I would recommend creating a bootable OS installer thumb drive and given the system you should stick with Sierra (HFS+ file system) or if you need something newer I would go upto Mojave (APFS file system) but that will require DosDude to run. High Sierra has issues! Format the USB with a GUID partition map and a Journaled File System before setting it up following this guide How to create a bootable macOS Sierra installer drive
