I think anyone even thinking about crusing or going offshore should be required to participate in a cruising rally to Bermuda first. Preferably from New York. I think that should be required as a matter of law.
Kidding aside, I don't think you can do this in the way folks are looking at it. I think you first need to start with the budget. Actual dollar amounts. From there you make choices. One of those choices by the way could include working for another month or year until you have enough money to buy things you view as necessities.
Plus, unless you have more information on what the plans are you can't have this discussion sensibly either. That is, what you need to live on the hook and float around the Florida Keys will be very different than if you want to do the same thing in the northeast, or if you want to do it in the Bahamas, or if you want to cross oceans, or be in the Med, etc. Posing the question this way is like asking how much money does one need in life? You can't even start the discussion unless you also ask/know, "for what?"
And the notion of being unable to afford a handheld GPS, to me, means you can't afford to do this activity. No chance you can own a boat, go cruising, and live life if you can't afford $150 for an item that has been the single greatest safety improvement ever to hit the boating world.
Just my view. Don't mean to be a downer.