I think you should make the track a bit bigger, and go with the Slash....18th scales are harder to drive, and if you have some jumps and such, the Slashes will be fun to watch as well as to drive. You might attract others to get involved, and maybe donate some more cash for more trucks, and other parts and such....

Another idea is since it's for a church group maybe you could check with a local shop or distributor and see if they'd offer you a discount, or maybe some free gear.....You might have to put up an advertising banner, but a couple of banners would ad to the atmosphere...

Good Luck