Welcome to the sport Sam. It seems that you have done some good initial research but as always there are some things that you can’t find out there.
The best thing is to hit one of your local tracks on a race day and talk to people. Not familiar with the ones around the Denver area but I’m sure someone else can help you on that front. It will be hard this time of year though as the season is pretty much over. Still try to contact someone here who lives close or a local shop.
TAG is a goof place to start for someone with your experience and it will give you the most speed without going into a shifter for which the learning curve would be quite steep. They have plenty of power to make your drive more technical and at the same time teach you the fundamentals.
As far as Chassis everyone has diferent preferences but most would be good choices. The general consensus is to get something you can get support from a local shop or someone. You can usually get a good used package for 2k to 3k.
Hopefully I answered some of your questions. let me know if you come up with more.