Welcome aboard Rod,
always good to get another Aussie on board (I'm Scottish, but based in Tasmania for 10 years+). there are plenty of good boats for sale
up & down Australia's Eastern freeboard. Don't hurry your purchase
... get a feel for the market and "kick as manuy tyres" as you can before you make your decision.
I haven't sailed a Martzcraft, so I can't speak from experience, but they have a good reputation as cruisers, and are, from memory, a Bruce Roberts
design, so they have a decent pedigree - Roberts is known for designing good blue-water dependable boats.
Other designs that might be of interest to you cuold include Joe Adams
- the 40' centre cockpit
version is a great crusing boat, although perhaps a little more pricey than your preferred range. I also like the Sparkman & Stevens 36' and 34' boats. They are renowned high quality boats and have been single-handed round the world
. There is a nice looking S&S 36' for sale
in Mackay at the moment.
Bear in mind that a 40' boat is a lot of boat to sail single
handed, especially if you ain't no spring chicken (no offense). If you are planning to live aboard and single-hand, I would be looking to the 33-36' range for ease of sailing rather than a bigger boat, which will have more space, but which might be tougher to handle by yourself. I speak from experience, having a 40' boat, and I can tell you its hard work getting it in & out of marina berths solo, but easy with 2 people.