It sounds like you are very new to Linux in general. Linux is an Operating System, like Windows and Mac OSX. Kali, is a specialized Linux OS with a heavy focus on IT Forensics and Cyber Attacks. This is not a piece of software, it is a full blown Operating System. Considering the relatively low hardware requirements to run most types of Linux, you're able to install it on a lot of different machines, including older, less powerful computers. For this, you could pick up an older, used netbook with at least 25GB of storage and 1GB of RAM and you'd be able to run the system. Some programs and task might take a lot longer to process, such as running Crunch or HashCat which run through innumerable calculations per second and better hardware will speed this up.
You don't need to buy a laptop that has Linux already installed, any device will do provided you have the bare minimum requirements listed in the Kali hardware requirements info. As a Linux OS, during installation (similar to installing Windows) you do have the option to multi-boot between your current OS and Kali, erase everything and have only Kali, or set up something else (use this only if you know what you're doing), depending on what you prefer. If you haven't had much experience with Linux in general, I would suggest learning to use a popular Linux OS like LinuxMint, Debian, Ubuntu, Zorin, etc. Basically, anything except Kali before taking the plunge. They all can be installed like I mentioned above, or run entirely from a USB stick without installing them. Do your research before diving straight into Kali; this is a specialized OS full of tools meant to be used for ethical, LEGAL hacking techniques.
I don't mean to discourage you, but if you're unfamiliar with any form of Linux, I cannot stress enough how much you should familiarize yourself with it before using Kali or any other cybersecurity oriented OS. At the very least, you could end up in trouble for breaking some lesser known laws in your area or lose a lot of data. It's happened to me in my early days as a Linux user. If anything, I know there's crash course videos for inexperienced Linux users, and even some more informative ones related to cybersecurity. In any case, I hope this helps to some degree, and I wish you luck on your journey. But I will say, don't dive into the deep end straight away. This is not an easy path to follow, and even the best of Security Specialists with 20 years experience are still learning. Good luck.
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Just because something is illegal, doesn't make it wrong. Just because something is legal, doesn't make it right.