Like people are saying, it depends completely on the sort of work you plan to do and what kind of images you want to create. For a bit of fill light or relatively simple setups where there is unlikely to be a high volume of images shot in a session, one or two 600 EX-RT speedlights would be handy.
If you need greater control and need to use modifiers then mains powered monolights are the way to go. Einsteins from PCB http://www.paulcbuff.com/index.php
represent extraordinary value and have been absolutely selling their socks off for the past couple of years since they started shipping. A more budget solution from PCB are the venerable Alien Bees. They're all explained pretty well on the PCB site or on the PCB Forums http://www.paulcbuff-techforum.com/
Maybe a carryover from your video work will be just as relevant for stills, gutsy LED lights and thoughtfully used reflectors, particularly as your 5D3 is such an awesome high iso performer.