Maintaining speed and getting up to speed are two very different things. Your motor/controller/batteries should be capable of atleast the "maintain speed" power level on a continuous basis, and a peak power level of your desired acceleration rate plus a little extra so you aren't pushing it to the max every time you accelerate.
There is a great excel sheet, I'm sure I found it in the wiki that lets you put in weight/drag acceleration rate, it gives you power required, power to maintain speed etc. I'm sure it's not perfect but it looks like it would give a good ballpark number as far as power and all that goes.
It's called evdrivepower.xls
Hope that helps
