But some poster have said that Linux is also infected, or something, by Microsoft or "them?" I'm not doing anything illegal but still hestitant to online bank or do business stuff on it. .
No, Linux is not "infected" with MS. If you run the version of Ubuntu that MS plans to layer on top of Win 10 that might be true. But if you install it and run it in the usual fashion, it has nothing to do with MS.
I can't find, maybe built in, anti-virus/malware programs for it and not sure about firewalls (not a computer guy so I might be not looking in right places.)
There aren't really any legitimate anti-virus/malware programs out there for Linux, as they are not needed. The explanation is a bit technical, but briefly stated, the Linux OS operates in a fundamentally different way from Windows. Viruses and malware that can infect and live within Windows will not work within Linux. That's one reason why Linux is such a popular choice with IT guys at the server level.
Not sure which version of Mint you are on, but somewhere in the Menu there should be a program called Firewall Configuration (look under System for starters). After you launch it, you need to unlock it using your root password. Then you can configure it. A quick, safe configuration is Status = ON, Incoming = Deny, Outgoing = Allow. You may find that this interferes with certain functions (for example, your torrents that are downloading horse porn suddenly dry up). In that case, you'll have to create a rule to allow that traffic past the firewall. For the usage you describe, these settings should work.
Did find bleach-bit, before the Hillary stuff even, but even then it "says" some stuff "errors" when cleaning. And weird stuff like "as root" or otherwise. Basically I'm paranoid a bit, and like to clear everything and not due financial stuff on it unless clearly secure, and not familiar with Linux and the system. I like the speed though..
Bleach Bit has a bad reputation within the Linux community and I recommend that you do NOT use it. You can actually do more harm than good with it.
As far as residue, there is much more of it in Windows, and even things like CCleaner don't clear out all of it. Linux is pretty efficient at keeping things tidy, but there will always be traces (thumbnails of your horse porn, for example). The only sure way to avoid this is to install Mint (or some other live boot Linux distro) on a DVD and run it off of that. You will be able to check email and watch your horse porn, but when you shut things down it all disappears. Nothing is written to the DVD or to your hard drive, and all traces go up in smoke. Alternatively, you could laboriously comb through your file system and manually remove traces, but even then you can't be sure. Certain files which you can't directly see into may contain residue you would want to remove (the pagefile in Windows, for example).