Great idea. I hope it all works out.
A chap who crewed with me many years ago had circumnavigated Aus. on a GBE catamaran
. He was extraordinarily modest about his achievement. He did say that he often slept on beaches and spent time in unusual places. This was before the days of GPS
, mobile phones, internet
etc. What I do remember was his sheer competence when on my boat. I learnt much from him. He had tertiary qualifications in marine
biology so he really knew what to look out for in different places.
Although he joked that navigation
consisted mainly of "keeping Australia on your left side" he was an extremely competent sextant/paper chart navigator.
So what those above have said is all true. It can be done without investing in an expensive vessel. It must be one you can easily manage by yourself.
You have asked "What makes a sailboat sea worthy of the open ocean? What features should I be keeping an eye out for?" I think that ultimately the answer is "My own competence." Your experience and accumulated skills.
However, the weather
patterns really matter. You need to invest considerable time in looking at the prevailing weather
patterns and local tidal rates (some of which are "exciting" e.g. Cape Don, Lacepede Islands).
If you use the trade
winds to Darwin and then visit the Kimberley's you may find yourself facing the SW winds that roar up the WA coast (after rounding Cape Levique.) And it is a very long coastline indeed. A potential hard slog with a shortage of stopover places.
My own advice is make sure you do not put a time limitation on the adventure. There are some absolutely fascinating places and peoples particularly after you pass Cooktown (often described as the largest outdoor asylum in Australia ... which alone makes it a must see place. The James Cook museum - in an old convent is just fascinating.) Another acquaintance (a couple) spent nearly a complete year in the Gulf of C.
The further you get away from suburbia the more likely you are to meet some really different characters. Dinah Beach yacht club in Darwin has it all. Hedonistic, frequented by international cruisers and a collection of independent life on the edge types. Nothing like it elsewhere. Possibly the Gove yacht club.
There will be many readers who just wish they could do what you are planning. I'm one.