We based our attempt on the Legends in America (
www.600racing.com) A great idea, very quick and reasonably cheap. Our car comes in at around 450 KG with 170HP. Absolute rocket. The hardest thing we have found is the official c--p that we have to put up with. After going through the motions I would have to suggest you use an existing category. Tracks in OZ are reasonably priced at around $3500 to $4000 for the day, however insurance doubles this. If you can get a decent field ie 20 cars this brings a meet down to around $500 per car which is very reasonable.
Keep in touch Im really interested in how you go. It sounds like your doing exactly what Im attempting in oz.