Liquid ring pumps with mechanical seals are about as low maintenance as you can get. As long as you don’t run seal dry, over/never grease the bearings, or run hard solids through it you’re good for years of industrious usage. A full overhaul would be bearings for the pump/motor, lip seals, a tube of flange sealant for gaskets or orings, a ptfe valve plate, and mechanical seal lapping or replacement. Very few moving parts. Total abuse and complete lack of or incorrect maintenance will be the thing to create downtime.
They make as small as a 5 cfm and go up to tens of thousands of cfm. That covers just about everyone.