Sorry to bring back the dead here, but just tested an idea to get that Speed 12 on your third day of racing without entering out of class cars where they don't belong (ie the 90's preludes in the 80's sports car races.)
Start with the Silvia K's 1800cc '88, checking for one under $3,800 after each license test, though $4,200 will do just as well really. Add the carbon driveshaft and weight reduction 1 and the best flywheel you can while saving $1,000. Now your first race is the first 80's sports car event. Should be close, but definitely winnable. Sell the Mugen Ferio and you should have $13,500+. Put on a turbo-2 and go win the last 80's sports car event at tahiti, should be easy if you've got at least modest skills, even without wall bashing. Now you have that Skyline that can take the All Stars at Red Rock on day 3, and there you go. Fastest way I can figure to get to the Speed 12 while keeping your races in class.
I'm sure there are other 3-day strategies that would work too, I just wanted to demonstrate that there was at least one way to do this that was fairly easy. I remember how long it took me the first times I played GT2 to get anywhere close to getting access to real money, and when I searched for advice I heard a lot about the Special Event Generator, free cars from Golds to get you started, and strategies which involved at least a dozen races and tune-ups before going for the Speed 12. I think my method here is by far the easiest way I've heard to get fast cash, and I like its aesthetic of racing the '88 model in the 80's races to get there, too!
Edit: Looked at some more cars, first a bunch which theoretically can be got for $10K or less (apparently the game allows at least +/-36% variation on used car prices, so I guess even a car valued at $14K could be found for $10K.) In reality I just looked at the cars which would be at $10K with only a 10% discount. In order to discover the base value of a used car, you have to buy it and check its resale value and multiply by four, so the RX-7 Infini which can be sold back for $2,525 has a base value of $10,100. Then I started a new game to see what cars actually show up on day 1, and again on day 10 in case you're puttering around with license tests and still haven't bought your first car.
So, among the cars which actually appear on day 1, there are several bargains among the best performers (rounded up to nearest $100):
*Supra TwinturboR '91 for $9,500 is the strongest car available and unmodified can win a lot of races, including 80's-5 for the Silhouette, but it will be tough to control especially if you're actually new to GT2.
*RX-7 Infini III '90 for $8,900 can take a racing flywheel and do almost as well but will be easier to handle, highly recommended.
*Skyline GTS-t '91 for $7300 will get weight reduction 1 and flywheel-3 and do very well and is the best Skyline option.
*RX-7 GT-X '90 for $6,600 will take WR-1, Fly-2, and muff-1 and do very well and handle easily.
*Prelude 2.2 VTEC '91 for $6,000 will take WR-1, Fly-3, and muff-1 to be your best FF choice by far.
*Supra Turbo '93 for $6,100 isn't the best bargain but will still show a lot of power and with WR-1, Fly-3, and muff-1 will be one of the strongest cars in this group.
Celica GT-R '91 for $4,100 is another decent FF at a great price and with room for upgrades.
Infiniti '90 for $3,900 won't disappoint you and is super cheap to upgrade.
*Silvia K's 1800cc '88 for $4,100 will take turbo-1 and fly-3 to be probably the strongest option available with both power and easy handling, 80's-5 definitely winnable!
CR-X VTEC '91 for $6,200 isn't the best bargain but is easy to handle and will get you winning quick.
Mazda 323 GT-X '92 for $4,700 is a 4WD option that won't break the bank though it's not the best performer of this group.
If you wait til day 10 to buy, you'll lose some options but others will open up:
Civic Si '93 for $8,500 is one of the best FF's in the game, slap a fly-3 on and all 295hp races are within reach.
*Silvia K's 2000cc '91 for $7,300 is another one of the strongest options available and with a longer potential career than the 1800cc.
better price on the Prelude 2.2, $5,600.
better price on the Silvia K's 1800cc '88, $3,700.
I've seen '91's raced by the AI at the 80's races regularly, so should be a legit 2 day strategy to get the Speed 12 to take the Supra Twinturbo, Infini III, GTS-t, RX-7 GT-X, or Silvia K's 1800cc '88 to 80's-5 tahiti for a day 1 (of racing anyway, after licenses) win for the Silhouette and then go to All-Stars 3 Red Rock for day 2 Speed 12 and $550,000 total after you sell it. Could probably also win at tahiti with the Prelude, though it would be the most challenging car available to try this with. At least in the Twinturbo when you fishtail and spin out you know you'll catch back up on the next straight with your 276hp... Supra '93 will also win at tahiti but isn't an 80's sports car, so...
