The HD series drills have larger discs and a weight package . THey came out about 2008 or so. That  said I have a neigbhor who no tills in cornstalks with his early 2000s regular 3S-3000. I have one also and you can get a lot of down pressure on them the older ones have a little smaller diameter disc openor but they are offset like the HD series. For the conditions you describe  I believe a  regular 3S-3000 would work fine. The weight package is just steel plates inside the frame tubes.