BIG DADDY

Hi, newby here! Say, I have this 94 sub. wagon of my daughters that we are selling but do not know what the "Mi" model stands for on the back tailgate, can anybody clue me in? The heater core is out, I have had an estimate from a mechanic to fix it for $600, does this sound high and how hard is it to replace? I live in Denver and do not really know any good sub. mechanics, any suggestions? Thx! BIG DADDY