Most are fine, generally more USB ports the better.
I guess it depends on what you're planning on using Kali for. If you are going to be hash cracking, you would be better off getting one with a decent graphics card installed, but I would have just created a live USB with persistent storage. Then you're not restricted to one computer.
I think most of the linux distros don't need high end system. If you are going to use Kali Linux and try some programming or coding any good laptop with 8GB RAM, Good Nvidia graphics card will help you.
I think 4GB RAM is also good enough with the Kali Linux. But it all depends on if you wish to run multiple operating systems at the same time for testing. In that case get i5 processor with 8GB RAM. Linux can run pretty swift with this.