Work out your hardware budget. Probably a good starting point. Good Allows you to have co-ordination between the music and the visuals. Mix and fade between two video channels. Speed up the tempo, or play slower.
Setup cost depends on the quality of video mixer.
Use live mixing effects unavailable on video mixers. Allows you program video sequences via MIDI or keyboard key strokes. Recommend scan converter Tview Gold. Give operator full responsive control over tempo and creative aspects. Argumentatively the best possible way for a VJ to perform.