dex will not influence crit.
3 main side stats:
Heal mastery, means your heal charms have 1ee more per mastery, not quite correct but close enough for a quick explanation
X Mastery (i.e fire mastery) that charm type will have 1ee more per mastery, not quite correct but good enough for a quick explanation
Critical Strike - Increases critical hit chance, casting already starts with roughly a 1 in 4 chance to crit anyway.
In general the less spread your charm has, the more consistent you will be for killing mobs.
As an example lets suggest you have a 80-120 damage ice charm or a 40-160 damage wind charm and a mob has 100 health.
The Ice charm would have a 51% chance of 1 hit killing it and 100% chance to 2 hit kill it.
The wind charm would have a 51% chance of 1 hit killing it and about a 5% chance it survives 2 hits.
This is partly why Ice is generally the most expensive charm.
Lightning is the cheapest partly due to this and partly because muters use this type of charm, so this would be the most common. Although being common is often a good thing as if you happen to use lightnings, getting a full set and upgrading specific parts is significantly easier.
My recommendation however is to buy whatevers got the highest EE for the cheapest price. Do not attempt to use the same type of charm for your first climb. Whilst yes it builds proficiency, getting 10ee more would be significantly more impactful.