First, choose a season length you genuinely want to complete. A shorter schedule can reduce repetition; a longer one gives you more events in which to earn money and build the car. Second, read sponsor contracts as combinations of signing bonus, advertised race payout, Showcase eligibility and brand discount, rather than ranking them by a single number. Third, distinguish facility unlocks from the cash needed to buy the parts or hire the people they enable.
Finally, keep performance and durability in balance. The strongest listed part is not automatically the most efficient early purchase, and an inexpensive intermediate upgrade is not automatically wasted money if it makes the next several races more enjoyable. The upgrade tables let you choose a route that fits your available funds and preferred level of challenge.