i seem to remember that when you get the dvd writers u have to make sure there all the same, im pretty sure of that. double check that before you buy.
that website will give u an idea of what you need to buy. from there try other sites or ebay for a better deal, if there is one
oh and one more thing u can get automated duplicators, i reckon a 1 to 5 is better then a 1 to 21 manual.
becuase i have experiance of using a manual Duplicator and although it does save a lot of time you have to come back every 15 mins to swap out disks, and when time is of the essence ........that does waste a lot of time.
alternativeley with a automated duplicator u can leave it on at night and let it get busy, i believe theres also models that print labels for you too. the only downside ofcourse is the horrendous price........