Download, try and purchase within the game (no other download necessary). The rFactor trial includes a selection of cars and tracks for evaluation. The full game content is unlocked after purchase.
With the introduction of version 1.255 or above, the trail criteria now includes unlimited use of rFactor for a period of 60 minutes. This should allow users to test all aspects of rFactor before making a purchase.
Once downloaded and installed, rFactor will prompt you to continue your free trial or purchase the game directly from that window.