I'm not an expert on Magic, but I can leave some input for you.
In the last 10 36 pack booster box openings I've done, in sets that have planeswalkers it's fairly common to get two per box and anywhere between 2-5 mythic rares. Also, every booster pack has at least 1 rare card (2 if you get a foil rare/mythic rare). When it comes to decks, the cost will depend on what play style you use, and as ase said, how competitive you want to get. My red goblin token deck cost ~$30 and can get really intense if things start rolling for me.
As for playing Magic online, you have this as well as the version on Steam. Personally, I think the physical Magic cards are a lot more enjoyable to play.
