I'd definitely go with a refurbished unit over a used unit with zero warranty unless you are familiar with mac and can inspect the unit for some time. PPC really is not worth it. Yes, you are looking for basic internet but you will probably use the mac for the next 7 years. You are then looking at the laptop your getting to be a decade or more older if you go PPC route. I'd definitely go with a refurbished unit if I were you. You can price the general range in the refurbished section at apple.com or even amazon (if within a year of purchase, it still has a warranty under apple). Then just keep an eye out for a good deal. You can also go on apple.com and type in a serial number of any computer you want to buy to see if it still under warranty.
If anything, buy the core duo or core duo 2 mac and be set for the next decade of internet. But to be honest, there so much potential in the net these days... Pretty soon you'll be watching hulu or netflix movies and therefore the "newer" (i'm talking core duo here) the better.