At first only the Android version will be available, with iOS version coming out some time later.
If you decide to buy the game between 5th of May and 5th of June, you will receive a Founders Pack. Said pack contains a bonus car as well as in-game credits.
Yes, Forza Street will have microtransactions. Ugh.
Interestingly, there will be an option to connect your PC profile to your mobile one. This means that you will not have to start your racing career from scratch.