The studio has released a handy comparison image showing the differences between the first game and this sequel.
The store page on the Neocore site lists six different purchase options. The cheapest, at just $15 (USD), includes a beta key, a copy of the game when it's released on April 17th, Occult Bargains Perk, Magic Helmet Skin for each of the three classes, and a Supporter Steam achievement.
The higher tiers include more and more bonus items in addition to the items listed in the tiers underneath them.