Channels: channels are number of speakers (or woofers) you can attach to your pre. 2 channel is for two speakers (either front in door panels or rear in rear deck). 4 channels is for four speakers, 2 front, 2 rear. 6 channel is same as 4 channel plus two channels for woofers
by voltage i suppose you mean pre output voltage. this goes anywhere between 0.2 volts to 6 volts. the higher the better as your amps need to amplify less to give you same output. amp input voltage is the volts an amp can accept.
the choice of amp depends on whether you want a woofer or not. if u want speakers only then any decent quality korean amp will do. for woofers its totally different and sky is ur limit