Yep, really doesn't take a lot to understand mining. Any ordinary Joe can understand the basics of mining, but with a CompTIA certification you're likely better off than the average starter.

It's absolutely okay to use calculators- everyone does sometimes for an approximation of break-even time, etc, but don't think that they're absolute by any means.

With $400, I would suggest looking at a GPU like the 1070, a decent motherboard, and perhaps 4GB of RAM and booting off a cheap, low capacity SSD or USB. The 1070s come for relatively cheap nowadays- heck, I just found one used for $250 + ~$12 shipping on eBay here.

As for mining less mainstream coins, they usually use more well-known algorithms so you can usually use a guide for mining another more popular coin if that other coin happens to use the same algorithm. Using a search engine like Google's extremely helpful too if you ever get stuck on how to do something like flashing a bios for a GPU, etc.