I would suggest looking in the 24-26' range. Something in that size can be found cheaply yet in usable condition. It will likely have standing
headroom for you, and with a decent layout can be sensible for coastal cruising yet still take care of you in a blow.
Everything on a smaller boat is cheaper.
Sails,
Rigging, anchors and chains, engines (on this size I like the Yamaha 9.9 4-stroke outboard),
bottom paint, etc. I'd avoid
electrical systems wherever a mechanical one could do. If you're plan is to sail in warm climates you won't need hot
water (the surface temp of the
water in the Keys regularly hits 90 degrees) or a
heater as badly as you'll need a
bimini (and a whole-boat awning, if you'll be sitting at
anchor for a while) and a well-insulated ice box.
Realize, as you seem to do, that this is an intermediate step towards your goal of long-distance cruising. It's another "learner" boat, much like the Potter is/was. The
next boat after this one is the real cruising boat, so leave everything off of this one that you can do without. You won't recoup the cost when you sell it a few years later. In a pinch, you could go
live-aboard on a 24-26' boat....
I'm partial to the Seafarer 24 (well-built and has an excellent
interior layout), though I've since
sold mine and moved on to something bigger for eventual long-distance use. My "something bigger" (a 1974
Cal Cruising 35) is structurally sound and has almost every imaginable system... but almost none of it works. The discount for this was fantastic! It has almost an identical layout to my old 1974 Seafarer 24, only on a much grander scale.
If you were to buy something like this some place warmer, get a job and live there for a year, you could get resident rates at the colleges. This would allow you year-round sailing opportunities (great for earning your captain's license) . However, having access to all those tools in your fathers shop and a lot of time that you
can't sail the boat each year, would certainly allow you to get more done. You'd probably be a lot further along in two years.