Angle grinder is the most versatile tool. http://kartfab.com/go-kart-plans/4-ways-cut-steel-go-kart-frame
Taken from Kart Fab: Angle Grinder: This is the one tool to have as you can use it for the entire build, it is cheap, and does the job of all of the above tools, plus it can sand, grind, polish, and scuff up the metal surfaces. It cuts fast, and wheel attachments are cheap. I used a 4 1/2″ angle grinder for almost all of my cuts.
Also I imagine it is not the best quality but... Harbor Freight Has one on sale for $11.79
Check out this Entire site. It is a great overview of Building a go kart. http://kartfab.com/