Prep? By prep u can call Konan's billions of explosive bombs. She had them in 1 place and couldn't move to another location to make it work. Orochimaru and Kabuto carry talismans and already prepared Edos in coffins because that's how they fight - with Edo Tensei. Saying ET requires prep is like saying FTG kunai, normal shuriken or senbon, sealing scrolls/tags or explosive tags require prep.
@Bold: Lol Edo tensei is a jutsu which requires multiple steps & is not always prepared. Kabuto
prepared his army for
war, in a 1v1 kabuto is not carrying an army of edos on standby, therefore he would need sacrifices in order for the jutsu to work during a battle. The jutsu is not prepared unless kabuto or oro know they're getting into a fight, then having prepped edos could be viable.
As for the talismans, just because oro & kabuto have one of the pieces doesnt mean that they have everything in place in order to summon & use an edo instantly. Let's hypothetically give kabuto a sacrifice & a prepped edo, summoning the edo & adding in the talisman is still one step too many in quite a few situations. Sensors, fast characters, & powerful characters can all get rid of the edos or kabuto himself with this kind of opening.
As for oro, prep or not hashi breaks free & the rest put up a struggle for control, so oro leaves himself wide open for attack.