I rode with Jeff on a pre-drive of the route today.
I'll make this simple: If you own a vehicle that you don't mind driving down some dirt roads, ENTER THE RALLY. If you're short on ground clearance, ENTER THE RALLY and skip the "extreme" stage. (yeah, you'll lose for skipping a stage... but trust me, YOU WILL HAVE FUN) If you have to rent a car, ENTER THE RALLY.
I'd estimate the roads as about 2/3 dirt, and a hoot to drive on. Only the one stage is really "challenging" to drive, and I'm sure if you enter and tell Jeff you don't want to run that part, he'll find a way to route you around it so that you can still enjoy the rest of the rally.
I'll put it this way... I don't think I'd have any trouble getting my Miata through this Rally course (except for the one stage... and even that one... maybe).