Early cheap and effective, just keep the bad guys at an arms length, you don't need everything. This will also serve as a base for all your future purchases
More expensive Focus on getting your Spirit Power and fire rate up as fast as possible. Keep in mind that you wont be able to get everything in here so prioritize getting items centered around what you're currently building for.
Late game Expensive but absolutely vital if you want to have any late game impact. Boundless spirit is extremely strong for the extra Spirit Power and fire rate. Glass Cannon will help you pass the 200 bullet damage mark.