OK, thats alotta ground to cover, so this is my recommendation.
For ALL things electrical, google 'Uncle Abdul'. Read everything you can by him, then come back here and see how you are doing, and what questions you have.
No shock (and no pun intended) that I agree with Ron. I'm more familiar with the wand than the tens. (Just a matter of personal preference.) However, sitting here and typing out everything that I know on the subject would be something of a chore when the information is already available. If you have a specific question, I'd be happy to address that.
I agree with looking up Uncle Abdul. But I have to say I have probably tried every electrical toy out there and done so without any problem. It is important to note that I am ok with the risk and have a strong heart. I also know when to say STOP and make it obvious that I mean what I say. If I play in private with a partner that enjoys the toys as much as I do, I need to know if they are CPR certified (I am) and that they care enough about me to call an abulance if that should ever come up.