Ensure the car/tracks you plan on racing are available for use with AI
here. If your main objective is racing AI be sure to check out Trading Paints to look for AI Rosters people have created, or you can create your own. I have created several, no NASCAR (sorry) if you like to check them out
here.
It is more than likely you will wind up doing official online races at some point and there are some good videos on YouTube to help guide you through the license process and best methods to purchase content.
Quick tips on saving as much money as you can. Take advantage of the initial 2-year subscription sale price. Buy content in batches of 3 or more to get additional discounts, 3-5 pieces = 10% off, 6 or more pieces = 15% off, 40 or more (yeah right) = 20% off. Check the annual Black Friday sale to get discounts on yearly subscriptions, you can add years regardless of how much time remains on current subscription.
Quick tips on advancing your license, race clean, avoid the stupid and worry less of winning and concentrate on finishing.
iRacing recently made buying content a bit easier to understand saying you need to purchase one of the following cars to participate in an official series, opposed to having a list of cars which made it seem like you needed them all to race (not the case). Similar to Raceroom where you only need to own one of the cars in a series to race, you see all competitors' vehicles whether you own them or not.
Good luck.