I'm wondering if the GNU License allows me to include the install file of any Linux Mint release as I wish in hardware that I'm profiting from?
Yes! You just need to follow the terms of the GPL: provide a copy of the license text, and either a copy of the source code, or a written offer to provide the source code on request. If you're distributing an unmodified copy of the installer, you can probably get away with just a mention of its source (e.g. the license and a link to where you originally downloaded it), but you might want to contact the Linux Mint development team to double check. I'm sure they'll be happy to hear you're including Linux Mint.
Yes it does. Also, once you show Windows users Linux Mint they are often eager to buy. We did a used computer sale as a fund-raiser for my work (We had a bunch of old Vista machines that nobody wanted). Out of 17 machines sold 13 of them were Linux Mint machines and only 4 still had Vista on them.