There are 28 upgrades in total so you don't have to buy them all, but will easily have enough to do so if you are planning on finding all the chests. Money is earned from killing enemies and opening up chests. As long as you don't skip lots of enemies this should come with natural play assuming you are finding some chests as well.
The Intenso ability is not available from the start but becomes so pretty early on. It is also recommended that stun boost and the Intenso shield breaker be some of the first purchases you make to make you're life easier.
The total cost for all upgrades is right around 50k.