I'd say at least 15 for each of them. If Attack is low, it doesn't matter. Speed too, since Ampharos won't be outspeeding much anyway. If you want IVs for HP Ice, I'd suggest looking up a Gen 6 Hidden Power calculator, or a set of tables for the same. It'll tell you what combination of IVs you need.
IVs can increase a stat by 31 points at most. It's pretty important that your IVs don't suck, but as long as they're decent, it shouldn't be too much of a problem. What IVs have you managed to get for Ampharos so far?