Advice on Class Progression and Car and Track Purchases

Hey everyone, sorry if this is a dumb thing to ask, I'm new to iRacing and I heading into C Class for Formula racing next season and I'm eager to race in the Indycar and/or F3, however because I have to purchase the cars and tracks and I have no knowledge on how it's like to race in either series, I was wondering if I could ask for advice when it comes to choosing a series and if it's better to buy most of the tracks for the series or just buy one or 3 because I'm really not sure how I should go with the progression and which particular tracks I should consider buying.