You bloody Teaser John
But hang on guys, I think you are getting a little carried away.
Have September as GP2 only, because 15 Sprint2 on track is quite sufficient, then at a later date, maybe even next year do an international open, no rules, all the factory teams will represent for sure and it'll be an awesome spectacle.
What you don't want is the two classes fighting for rostrum space. You certainly don't want to have to turn GP2 teams away for the sake of having maybe three GP1 cars running around, would be just daft. Besides, what's better than one 24 Hr event? TWO 24 HRs Evenets ;-)
For the GP2 you need a really slow sensorless brushless, something like 17.5 would still be bloody fast compared to the 20T Saturn. There are several systems on the market that would work very very well and the whole combo, motor ESC and card comes in at uder £70 and I'm sure you'd get a deal on that if you bought in bulk and got the supplier involved to sponsor it in order to promote their product and prove it's reliability.