Save up for a new one. Xbox 360 will probably have a price drop and and upgraded hard drive coming soon if the rumors are true.
I would recommend buying an 'inner cooler' when you buy yours because they have a tendency to over-heat and break. I have an inner cooler and never have any of the problems many people seem to report.
If you buy the 360 new there should be a guaranteed warranty from Microsoft (free of charge) so long as you don't modify the console, so don't let the store sucker you into paying for one. But, I would still ask if the console has a Microsoft warranty just to be sure.