#1
What should I be looking for,

Looking for something with delay, compression, noise gate? etc

Sound is for a basic metalcore etc sorta band
#3
You could get a brand new Boss ME-25 for that much, and probably an ME-70 for that much used.
Are you looking to have the multi-effect go through your amp or is it just for recording into a computer? Or something else?
#4
I was thinking maybe the ME-25 but how many memory banks does it have?

I assume the ME-70 has more memory banks to use?,

It will be used with a 6505+ so probably through the effects loop of the amp.