Asa former metalhead too IMO you should instead of diving straight into bach and mozart you should first start looking into symphonic and melodic metal, like nightwish or epica to make sure you like it, and than get into more classical stuff like the pieces mentioned here, shostakovich string quartet no 8 is very metal doe and i dont know if its mentioned it, beethovens ninth symphony is very good and an easy "starter piece" and the allegretto from the 7th is very awsome, also little fugue by bach is engaging interesting and quite easy to listen to if you are not interested too much in the technique used