Wanted to get this out quick so apologies for the excessive edits and the meandering. Quick rundown:
1. Career mode (rally/rallycross) is the easiest way as a beginner to make money (and to unlock car engine upgrades) without having to wait for a weekly/monthly event to expire. Each difficulty increase nets you more credits, naturally.
1A. The Daily/Weekly/Monthly AI challenge is technically the easiest and fastest, but it is locked to whatever difficulty you are on in career mode, you need to have the appropriate car, and you HAVE TO finish the event in its entirety, else forfeit any winnings.
2. Completing 1 or 2 stages in a weekly or monthly event (without crashing) will net you at least 100,000/1,000,000 credits when they expire. Great way to build a bankroll but requires some patience and the necessary car.
3. Upgrading your co-driver and chief engineer can help reduce time penalties and speed up repairs. Worthwhile if you find yourself going off the road often.
4. If you start a car in one event, you cannot use it in another event until you complete the first one.
5. There's no real secret to Career mode - pick your tires, drive within your limits, finish the stages without crashing, rinse and repeat. Don't get caught up in getting first every time, just keep at it and you will beat the AI in no time.