I have about 75 cars on my garage, didn't spend a single credit, except two "homologation special" cars bought using miles (you soon will realize miles are a far more important currency to grind in the game than credits). I play about 45 min most days, a little more on the weekends.
The prize cars are sort of random, honestly about 25 of this are duplicates. Have also about 6 millions (European currency)
But on this cars I have a Peugeot 908, a Toyota TS050, a Porsche 919, a Mazda LM55 gr.1, two SRT TOmahawks (a gr.1 and a gr.X), many Gr. X cars, a handfull of Gr.3 and another handfull of Gr.4, two Gr.B rally cars, etc.
1st - Go to Campaign Mode, Racing School is pretty easy and quick to make through the 40 tests, each series of 8 tests gives you a car, so 5 cars, another one for "all silver" and finally one for golding all the tests (so 7 cars easy).
2nd - Circuit Experiences - Each circuit has time trials both for sectors and also for single lap. The smaller ones are very quick to complete, each circuit gives you a prize car, but you need to gold all tests (which is fairly easy in most circuits, to be honest, the target times are pretty standard). Some tracks can be more time consuming (Nordschleif is about a dozen tests, including a full lap, and honestly didn't tried rally stages, no idea how easy they are), but most of them are pretty quick (every fictional track, Brands Hatch (both Indy and GP), Big Willow, Nurburgring GP, Interlagos, apart from Mount Panorama where I found a little hard to gold the middle sector with all the SS downhill), this can easily give you about a dozen cars more.
3nd - Challenge Missions, 8 blocks of 8 challenges, a little more difficult than racing school, from the Stage 5 on there is at least and endurance race, so from stage 5 the progress is more slow. Each block gives you a car, without need of gold. So, at least 4 cars are pretty easy to add to your tally.
This challenges net you also very good payoffs in credits, miles and Xp. So, you will end up about 22-23 cars, at least a couple of millions in credits, thousands in miles and the Xp will allow you to go up some levels (whom is required to unlock the tracks, f.ex. Nordschleif requires you level 20).
Also, each day, completing about 26miles/48km, you are awarded a prize car. So, not spending huge ammounts of time playing, you can easily add 30 cars to your garage (expect some duplicates) in a week, without spending a single credit.
Other thing is that you can "sign contract" with one of the manufacturers (except those who haven't gr.3 nor Gr.4 cars, like Fittipaldi, Infiniti, Mini, Daihatsu or Alpine) present in the game for free, which results on the manufacturer loaning you one of is Gr.3 and Gr.4 models. Only thing is that you can't edit liveries on this cars.
