Dual boot.
If you want to "really" get involved with Kali then you need wireless functionality, in my travels with virtualbox, whilst it is brilliant, it doesn't pass a wireless chip through as a wireless chip, just ethernet.
128GB may be a bit slim for a Windows AND a Kali install and I'd argue you'd be better to buy a cheap second hand laptop to run Kali on than paying through the nose for a bigger SSD. Kali is debian based so it will run on very minimal resources.