From what I've read, OEC as others have mentioned is the bare minimum, and in many cases WFR (wilderness first responder) is expected if not required. That said, if you have the time and money I would go for a full W-EMT course, as that will give you both the wilderness component needed for guiding and the standard EMT component if you were to want to pursue things further - I've heard that a number of former outdoor professionals transition into paramedic and EMT type positions later on. That said, they also make it pretty easy to upgrade, so if you only go with the WFR now it'll be easy enough to do the EMT component later.
It's a international system from what I can see so I'm sure you can find places closer to home to take it, but I know in North America NOLS and IWLS are pretty respected for the first aid training, as is Rocky Mountain Adventure Medicine (although they only offer up to WFR).