I can partially answer this question myself. The iPad's basic mail app Mail can display HTML-formatted text, but it can't originate (author) that text itself. What I do is to use an HTML-editor app, compose the formatted email in that app - where I can control the font, font size, font colour, formatting etc and then cut and paste the resulting formatted text into the iPad's native Mail app, where it displays just fine.
It's very straightforward. There may be other better mail apps out there that members can recommend but there are certain advantages to sticking with the iPad's native Mail app.
There are several HTML-editors out there in the App Store, depending on precisely what you're looking for.
Tim