Eventually that sand filter will plug up and its efficiency will diminish. You need a way to backflush the sand filter.

One alternative might be to purchase a sand filter from a pool store. If you have a pump and filter it will run on 120VAC and the valve mechanism allows backflushing of the filter.

About the heavy metals - keep in mind that they could be disolved in solution and difficult to filter out, though it would seem most would come from particles floating in the air, which could be filterable if filtered out quickly after a rain fall.

What kind of roof are you planning to collect the water from? It would seem that a electrostatically painted metal roof would be best.