Welcome to karting. You’ve found a great resource here on the interwebs. Also check out ekartinews.com for other ‘new to karting’ information.
Ref your question of which motor and chassis, Rotax is ONE answer, but definitley not the ONLY answer. Tag is great because of the variety of motors allowed. Enjoy your shopping experience.
Ref your question of which series to run, for your first year, I’d stick to club racing. The CSC (Colorado Sprint Championship) is for those who might be looking for a higher level of regional competition.
Keep asking questions, we’ll help you out.