If you do think you have something of value, I would always make sure that you speak to somebody who is an expert, because I have heard stories and seen things where people have undersold things grossly
Madison Li's effects use mana to cast, so as long as you have the energy counters to pay, you can use any of them depending on what you need for the gamestate
Can you think of a way they could be brought back into modern Magic
If you choose to do so, you cast it as a spell
Charizard 4/100 - EX Crystal Guardians
Vadrik, Astral Archmages text reads, Instant and sorcery spells you cast cost X less to cast, where X is Vadriks power. This means that you pay all the cost of the spell (mana value + kicker) and then reduce the total cost by Vadriks power if you cast an instant or sorcery with kicker