I would start by stiffening the body, everything you can do/make yourself if your saving money: stitch weld the body, make your own subframe connectors, inner fender braces, torque boxes, lower rad braces, roll cage, and weld the all in (depending on what track your running look at their rule books for the allowed mods you can do to your car first).

weld up and gusset your kframe as well.

This is where I'd start first before you buy any aftermarket parts, this is so what you do end up buying can do their job efficiently.