To answer your first question, ALL of the mirrors are safe. The idea with mirrors is that you choose one that is geographically close to your location (same country, maybe the same region or state) to help reduce lag in downloading the files. Any selection is valid, all of the mirrors will have the same content, but for your location some mirrors may simply be faster or slower than others.
One other thing you might run in to is different types of installers. Some may be a very minimal install which only give you a command line, while others will provide a fully configured desktop. You might also see an option for a "Live" version -- that is something you put on a bootable memory stick and you can test out a working version of the operating system without actually changing anything on your computer, but all of your settings will go away when you reboot.
I haven't used Mint so I can't provide specific info, however some things that are common to ANY linux desktop -- You probably want to start with printer drivers (install CUPS) and some office software (install LibreOffice). For internet access, Firefox and Thunderbird are good choices. LicreOffice, Firefox, and Thunderbird can all also be used on Windows if you want to check them out ahead of time. There are many lists online that can point you to equivalent software, so like if you use Lightroom on windows, you might try Darkroom on linux. basically you just need to make a list of what Windows software you use and then install a similar package in linux.
Yes you will need some time to learn the new operating system and all the new software, very little will be 100% exactly the same, but they are "close enough". You figured out how to use all these things once before, so you can do it again, and it will definitely be worth the effort.