Looks like a 325. I have found the 325s to be good machines and I know a guy that as an older 425 which goes to the trail regularly. $250 for it is a great price and your boys will learn skills that they will use for the rest of their lives. I have said it before that there is no feeling like the first time it fires up and runs on its own....The 325s and other early machines are fairly simple to diagnose and repair. I have not found a specific manual for the 325 or 425 for that matter, just a generic Polaris ATV manual which covers it. From Haynes, covers 1998 thru 2007 I think. I rebuilt the engine in my first one following that manual.
If you search for your machine on Ebay, you will find both paper and electronic versions of a manual. I prefer paper myself, but the electronic version on a CD may be more specific to the machine.
bikebandit.com is another resource. They have the exploded parts views that can sometimes give you an idea what is missing or how it needs to come apart.
Good luck with the project!