very good idea!!!
if you're going circuit racing, the first thing you should buy is semi slick tyres, SAU membership and CAMS Licence...
then a fire extinguisher and battery and tow point stickers...
now you're ready to do super sprints and hill climbs
then in no particular order...
racing seat
coil overs
sway bars
camber and castor arms
exhaust
oil cooler
radiator
intercooler
ecu
some gauges to keep an eye on oil temps, etc...