You want a machine that can run virtual machines. Look to get a minimum of 4GB for your system. 8GB is good. 16GB will run 3 or 4 decent virtual machines. Make sure you have a SSD disk and modern CPU (Last 3 years). The CPU should be optimised for virtualization, such as those with AMD-V or Intel-VT extensions. 1.3GHz or faster core speed.
You also need 250GB minimum so you have enough space to store the virtual machine guests.
If you want to try WiFi hacking you need a WiFi adapter that can inject packets and works with Kali Linux. These cost about $40.