I doubt they would expect you to know much about plants from the start; obvious bonus if you did, but of the two skills, I would think that first aid is likely to be more of a make-or-break thing.

For a paid guided tour business, it's pretty essential for them to have guides that are somehow certified (with up-to-date certification) in being able to care for their clients medical emergencies at need.

Perhaps there's a lower-cost more basic first aid class offered by your local city? Far different from an actual wilderness first aid class, and my last experience along this line shocked me a bit in how much an "urban" style first aid class boils down to "call 911 and don't do anything dumb", but still --- having any sort of current first aid credential couldn't hurt.