In case anyone still needs tips for GT3 early game, the most efficient start I found that doesn't involve golding licences, beating time trials or abusing the trade system is this:
1: obtain the B licence
2: buy the '93 Miata for 16,900, then give it an oil change for a temporary power boost
3: enter and win race 1 of Spider & Roadster at Deep Forest, set ASM to 0 and TCS to 1 in the car settings before entering (if you come 2nd, buy a racing flywheel and try again)
4: use the 2.5k prize credits to buy a racing flywheel, racing chip and weight reduction stage 1
5: enter and win race 1 of NA at Apricot Hill, if the MR2 / MR-S is in the line-up, exit and re-enter to reset the line-up
6: use the 5k prize credits for a semi-racing muffler and carbon driveshaft
7: enter and win race 2 of NA at Grand Valley
8: use the 5k prize credits for port polish
9: enter and win race 3 of NA at Apricot Hill, if the MR2 / MR-S is in the line-up, exit and re-enter to reset the line-up
You'll get a CR-X del Sol for beating NA, which is good enough to beat FF with little-to-no upgrades, and your Miata will be more than fast enough to breeze through Sunday Cup and Clubman Cup. The 5k credits from the third NA race can be spent on a stage 1 turbo, which will bump up the Miata's power an insane amount to 298hp. You can do Turbo with it, but it'll be pretty difficult to control until you upgrade the suspension, tyres or gearbox to reduce wheelspin. You can raise TCS to 3 or higher to compensate, though. It's also eligible for 80's Sports Car Cup. Finishing off Spider & Roadster will require the custom transmission in any case, since the Miata has a lower top speed than the SLK on Test Course (it's winnable with the stock gearbox, but it isn't very consistent). I elaborate more on all of this
here.
Doing NA as early as possible is great because each race pays 5k per win, and even if you don't win and finish 2nd, you still get 2.5k credits, which is more than Sunday Cup and Clubman Cup pay out for 1st place finishes. If you need to grind for whatever reason, the first Apricot Hill race only takes a couple minutes to beat and is really efficient at this point in the game, until you can do something like European Championship, where you can reset the prize car roulette so you win the Vertigo for 300k.