Sam, one other thing to consider, Parilla electric start systems have onboard charging, Rotax doesn't. I would expect the water-cooled Rotax to last a whole lot longer than an air-cooled IAME.
With that said, I run an HPV which is an air-cooled IAME motor and I love it! But it goes only 10 hours before I have to send it in for a top end.
As far as the chassis recommendation, that all depends on where you live and what track(s) you wil be running at. Either chassis is a top quality unit that will be a great choice. The question is who will be at the track or nearby to give you chassis support.
The same holds true for your choice of engine. I like IAME products better than Rotax, but that is only a personal preference. Both are quality products.
Tom Stephens
www.kartfinder.com