Electric potential

O.k. So, you would need to translate that potential difference (V) into the electrostatic force, which is simply dividing the V by R. Now, you have N/C, and have a charge and mass, so can get the acceleration in the opposite dirrection. I didn't think too much about it, but I guess you would integrate the acceleration equation with respect to time, and go from velocity = infi to v = 0.

Sounds like you know what you are doing at any rate. Good luck!