I would buy the smallest set (by volume) possible. Casting supplies have a limited shelf life- a year or so. I've never managed to buy resin and use more than maybe 1/3 of it before it goes bad. Most 'casting kits' are intended for making larger items (say baseball size). In car modeling, there's rarely the need to cast something much larger than cocktail olive or AA battery size.
I would happily spend $20 every year or so on a kit with 8oz of RTV, and 8oz each of a two part resin. But I never can get through a litre of the stuff.
