I think the general concensus is that the Ice is THE best bang for the buck charger. It charges NiCd, NiMh and LiPo, will run a motor to break it in and doesn't cost anywhere near as much as the GFX. I have had one for about 2 years and have nothing bad to say about it. As for learning about electric you have found the perverbial mecca. This forum has all the answers you could ever want. Just use the "search this forum" and "search this thread" buttons in the top right corners and you should get a good start. Or PM me if you have a specific question. I don't know it all but I can at least point you in the right direction.
Faster charge = more power(to a point) at the cost of battery longevity
slower charge = longer battery life and longer "play time" each time you charge at the cost of power.
A 15t motor should be a lot of speed if the motor is in good shape. You may need to rebuild the motor . . . or you may just need more speed
For that I suggest a brushless setup. I would suggest an LRP Sphere or Sphere Comp or Novak's GTB paired with your choice of brushless motors 4300 or a 5800 from Novak would be a good start to get you close to the speeds of nitro. Welcome to electric r/c!!