Since it's a used bike and if you don't know it's maintenance history, I'd do a complete overhaul including clean and relube all the bearings and components, lube all of the pivot points,fit new brake pads and new cables and housing. That way you will have a known starting point for future upkeep. Check the chain for elongation as noted above and, if it's within limits, just clean and lube it. Chainrings are very durable and typically last 10 of thousands of miles so it's unlikely they need to be replaced.

Any tire that's really at 100 psi shouldn't look "flat" so check to see if your pump gauge is reading properly.