In that case for larger doses you would be best to use a timing motor, I've salvaged a few from old dishwashers but they can be purchased cheap at surplus stores: http://www.sciplus.com/category.cfm?subsection=18
There's models from 2rpm to 2rpd (revs per day for hour hand movement). They are usually very strong due to the large gear ratio used. I always wanted to make my own pumps with these but I didn't want to bother with the control circuit to change the speed in order to change the dose. This design on the other hand can accomodate various dosing volumes by changing syringe size and off-center position of the plunger without having to change the motor speed. Much easier to realize and adjust IMO.