if you're going down the MP3 route have you considered a midi contoller? You'll need a soundcard with two outputs mind. With the Vestax that Swiss and I have you can route the two channels to seperate channels on your mixer. If you have a 3 or 4 channel mixer you can then mix vinyl to vinyl, vinyl to MP3, MP3 to vinyl or MP3 to MP3. It retails at about £380 ish but if you don't have the right sort of soundcard then it'll be another 70-120 quid minimum. Just an idea mate
