The closer to the minimum the monster levels are, the better. That prevents the generation of higher level affixes which improves the chance to get what you need.
You can play around with this affix generator to find the exact value. Try different values in the item level filter until the affix appears on the list.
Only one affix per group can occur on an item. It's only relevant for rare items (which can have multiple prefixes and suffixes), but there are no rare charms. Magical items have a maximum of one prefix and one suffix.
The "level" column means the affix level. It's used in the item level calculation. You can find the formula in the statistics forum, but you don't need it if you try out item levels until you found the minimum value for the affix in question.