That's an odd question, as a person usually picks their cowherd before their bull.
However Charolais are a lean, usually high performance breed, so you should have cows that will balance that. Angus or Hereford will cross. Herefords will produce a yellow calf, Angus will give you a grey one. A baldie cow (HerefordxAngus) will produce a tan white face with that cross.
You say calves will be sold at weaning, so you will want heavy milking cows. That will be on a case by case, but usually Angus milk better than Hereford. Baldies usually milk well if they are from good milking stock.
Carcass traits have to be considered as it will influence what price buyers will pay for the calves at weaning. Charolais have very little in the way of marbling, so the cows will have to carry this as well.
As I said before, usually you get the cows and then choose a bull to compliment what you have.
Jason Trowbridge Southern Angus Farms Alberta Canada