Don't waste all your coupons on the mission boxes now, that's a pretty dumb idea. I'd say just have someone give you 10 easy missions to do each day and from that just invest all the points into Med refines and wait for a refinement event to come along so you can get that refinement level up.
I'd recommend you wait for certain events where they give you good deals on materials that'd highly benefit you in power, because as a F2P you'll need all the power boosts you can get to be able to at least even reach top20 on your server.
this looks interesting can you give me an example of a good deal event?