Yeah, I'd recommend taking at least one finance/accounting class. Doing well in the class will demonstrate that you can handle the material, and (maybe more importantly) demonstrate that you're serious about learning the stuff that you'll need to know in b-school.
As for the GMAT, I don't know if there's an ideal time. Most schools want scores that are no more than five years old. So, obviously you don't want to take it too soon. But there's no rush to take it, either. You have a lot of time right now, so use it to your advantage. I strongly recommend studying in bits and pieces over a long time rather than cramming in the one month leading up to the exam. Steadily expose yourself to the material over time, and by the time you go to take the exam the questions will be so familiar to you that you'll feel very comfortable. This is what I did, and while everyone's learning style is different, it worked well for me. Usually people will respond "But I don't have that much time!" The good news for you is that you have a couple of years before you're going to apply, I assume.
Leadership experiences in the workplace and in the community are weighed fairly equally. All things being equal, they probably prefer a work-related experience, but a very strong non-work leadership story beats a merely good work-related story any time. When thinking about what types of examples to write about, use this as a general test: What happened/was achieved that wouldn't have happened if you weren't there? Were you merely part of the group? Were you just following orders are doing a good job? Or did you really make something happen, something that not just anyone else could have done?
Regarding Veritas, their instructors are really strong. (Note that Veritas is a partner of ours. We've partnered with several GMAT-related firms.) Veritas is positioned as the high-end course, and they deliver on that promise, based on what we've heard.
Best,
Scott
_________________
Co-Author, Your MBA Game Plan