Change font of a single word

hmmm... definitely not out of the box so far (as i know).
would assume it demands at least some custom css (or even more...) - better to use some plugin maybe. there has been some 'tinymce custom styles' or 'tinymce custom formats' -plugin somewhere that does the trick.

I wouldn't suggest this unless the other font is a system font.

Including too many Google Fonts will really hurt the performance of your site.

You're right though, you would use a span.

I'm the regular font, and <span class="new-font">I'm the new font</span>.

.new-font {
    font-family: Arial, sans-serif;
}

Are the font names in the customizer drop down box considered system fonts?
So this should work...
.new-font {
font face: lato;
}

That's a Google Font. It will only work if it's selected in the Customizer somewhere, or if you embed it using the instructions Google Fonts give you. You can insert their code into the wp_head hook in GP Hooks.