User blog comment:MegaDriver80Wiki/Which cars should I save gold up/@comment-25956839-20151013145628

That depends on your goals, this may help:
 * What are your goals in Real Racing 3? - Interesting forum post about goals.

Also if the car has been regularly on sale:
 * List of sales so far - Every car on sale since 2013 ! Useful for planning to buy an expensive car.

If you want to get to 100% completion, (although this will probably follow on from getting 100% of the cars first):
 * Upgrades Strategy For 100% Completion - Upgrade requirements and rewards to get to 100% completion

My progress may give some idea's too, although it was @ May 2015:
 * User blog:RR3 Michael P/My car garage, upgrades and progress though RR3 - my progress and cars bought (May 2015)