Please recommend a Flash for OMD

I'm totally an amateur to photography and know nothing flashes. I do, however, have a pretty knowledgeable friend that told me that in order to elevate to the next level, I'd need to learn how to use flash. I've also received a casual offer from a friend to shoot some models for a small web-mall that he runs. I imagine I'll need fill-flash (right..?)

So I'm thinking about buying a flash unit that's fairly versatile and leaves room for me to learn and improve my photography using it. Amazon keeps telling me to buy the FL-600R. Is this good enough? Or too good? FL-300R? Are there cheaper alternatives? Third party? Please help!

Thank you!