The demo for Forza Horizon 3 has been added to the Microsoft Store and is a 19GB download. 
The visuals are simply stunning and system requirements are:
Minimum
    OS: Windows Anniversary Edition 64-bit 14393.102
    CPU: i5 3570 @ 3.4GHz
    GPU: NVIDIA GT 750Ti or AMD R7 250X
    VRAM: 2 GB
    RAM: 8 GB
    Storage: 21 GB available space
    Expected Performance: 720p/30fps
Recommended
    OS: Windows Anniversary Edition 64-bit 14393.102
    CPU: i7 3820 @ 3.6GHz
    GPU: NVIDIA GTX 970, GTX 1060 or AMD R9 290X, RX 480
    VRAM: 4 GB
    RAM: 12 GB
    Storage: 21 GB available space
    Expected Performance: 1080p/30fps
Ideal
    OS: Windows Anniversary Edition 64-bit 14393.102
    CPU: i7 6700 @ 4.0GHz
    GPU: NVIDIA GTX 980Ti, GTX 1080 or AMD Fury X
    VRAM: 6 GB
    RAM: 16 GB
    Storage: 21 GB available space
    Expected Performance: 4K/30fps
 
 
My only criticism is that the demo won't save your profile nor your achievements so you have to start over on every play.  
Despite the above I still highly recommend checking out the demo, it's a blast!