I second David’s comment re: Briggs LO206 class. Its affordable and the formula will allow her to focus more on racing rather than tuning. Rotax would be fast enough that tuning the kart becomes a bigger part of the equation and that might make it less fun if she can’t be competitive.
As for chassis: at GoPro Motorplex I’d go with Arrow as they can provide a lot of support for the chassis there. And the majority of garage owners there have Arrow karts so you can get more advice for set-ups and what not.