30 dollars! A good deal for someone that likes working on bikes. Maybe you can ride it as-is, with just some cable adjustments, oil the chain, adjust the saddle and bars, and new tires (maybe new rim tape if any spoke holes are showing at the tape edge)

An overhaul on a bike like this is pretty straightforward. I've used Park Tool's repair web site to find out how to do stuff. But it'll require tools -- cable cutters, bottom bracket tool, cone wrenches, pedal wrench, etc. The costs add up for a one-time overhaul. You have to like doing stuff like that.