Very basic recommendations from someone who has lived through two Weber engine fires. One put out by a passing trucker, the other by myself a year later. Sometimes, I'm a fast learner! I bought an extinguisher for every car I owned before I fixed the damage from the 1st fire.
1)- Pick a gaseous or foam media extinguisher in 2-1/2 lb minimum size. Damage & clean-up from powder extinguishers sometimes exceeds the damage & cleanup that occurs from just running away and letting the sucker burn.
2)- if you go for a mounted-on-board fire system, don't make the mistake of aiming one (or more) nozzles at the driver's face! Its impossible to make any rational decisions during an on-board fire, with a faceful of extinguishent (according to a nationally known racing safety expert- not me!)
J DeRyke