To automatically install macOS updates in the future, including apps downloaded from the App Store, select ”Automatically keep my Mac up to date” from Software Update in System Preferences. The latest version of Apple's Mac OS X operating system is 10.8, known as Mountain Lion. For a list of system requirements for Mac OS X 10.8, see OS X Mountain Lion Technical Specifications. Note: Before upgrading, you should consult the manufacturers of any third-party hardware and software you use to make sure it will be compatible with the new operating system.
If you need 10.9 or higher then you do not have much option. I do not know how amenable Apple is to installing what they consider to be retired operating systems if you walk into a store. Presumably if they wanted to make it easy to get these older systems they would leave them up for download, and you need to have compelling reason to not go for the 'newest' and 'best'.
Yes, any new system, particularly one that is 3 upgrades higher than the one you have, is going to place more demands on the resources. You can try doing things such as installing a faster drive and more RAM if you consider that economical for your computer. There's also options such as installing 10.11 to an external drive and booting from that when you need to use El Capitan but otherwise run 10.8. A lot of the specific recommendations really depend upon the details.