Keep in mind that all of this doesn't have to be purchased at one time.
Granted, there are those that do the cavern thru full cave in 8 days but most instructors I've been in contact with recommend doing the classes in stages and gain experience at each level before progressing on.
This will allow you to start in cavern where your OW gear can be used and no canister light is required, no doubles required and such.
You can slowly accumulate this gear as you progress and of course if you decide it isn't for you early on, you haven't spent much.