The reasons that I pre-poo is because as we know the hair is weaker whenever it is wet. So by me strengthening/moisturizing the hair (with conditioner and hotoils) before I wet it, then i am giving it a little extra strength before i begin washing. Also, i apply oil to my ends so that when i am shampooing, i am washing away the excess oils, BUT i am not stripping my hair of the oil necessary for my hair's health.
And since your pre-poo trtmnt is rinsed out immediately, any cheapie conditioner will do.
hope this helps...