As above, what can you afford?
I have a mate who raced Caterhams, it was costing about £1000/event.
Initial car cost
trailer
tow car
tools
spares
tyres
brake pads
Also expect semi regular
Engine rebuild
gearbox rebuild
Wear rate is massive compared to road work, everything is getting hammered.
Also when it gets binned, if you cant afford somebody else to drive into the side of you and wreck the car in the first race....
Hillclimbs can be cheaper to get into if you are in a road class, also if there is a road class time attack series near you.
Still normally need
licence
suit
helmet
then your consumables
Being competitive can become expensive quickly, eg front runners with a new set of tyres per 20 min race! Engine rebuild every season etc.