Revert MacBook Pro from Windows to OS X

Hi all,

I'm looking into purchasing a MacBook Pro (2009-10) but the owner says it currently has Windows 7 on it. I can't get much sense out of the guy but worst case scenario, if the Mac partition has been removed and only has Windows 7 on it, how easy is it to install OS X back onto it?

I don't think it comes with any disks, it's basically just the MacBook and charger (it's only £90) but all I have at home is an iPad and a Windows PC - so I don't think I can create bootable media for it.

Is my only option to purchase a copy of Mac OS from somwhere and can this be the latest or will I need to find the one specifically for the year of that Macbook?

Thanks