most race sanctioning bodies require a minimum of 3 layers of nomex (or equally fire retardant material as allowed by said bodies). you can accomplish this with either a two layer suit and nomex underwear or a three layer suit. if you have any facial hair, most bodies require a head sock. add to this nomex gloves and shoes.
helmet should be a minimum of sa95 (an "m" rating is generally only allowed in open track events and "sa" is required by sanctioning bodies). however, an sa95 is only good untill the sa05 comes out. you would be better off getting one labled sa00. even if you have an enclosed car, i would suggest a full face with visor. more protection in case of fire or flying debris from the track.
you might want to investigate a head restraint device. i have used a hutchens device but have switched to an isaac device (
www.isaacdirect.com). i did this because i think it far greater protection and with it i don't need a neck ring as it seems to help support the neck as well.
there are a number of quality brands available, but don't go cheap when your life is involved. window nets in a coupe would be good. however, most don't want to go thru the trouble or expense. hence, arm restraints. these are intended to keep your arms inside the car in the event of a change in car attitude.
then you mention the full roll cage and there are a number of things one can address there.
a fire bottle? full system or just an extinguser? to which are you referring?