As mentioned above:
- Decks, if teak are a problem.
- When the decks fail, so to do the fuel tanks. Big $$$$
- Windows, when we bought out GB 36, the windows let light and water through with equal ease.
- Electrical, some years good, some years bad. Apparently, when ours was built, they had a special on red wire. EVERY WIRE, from the original build was the same shade of RED! The only non-red wiring had been post build.
You haven't mentioned vintage. I am assuming Fiberglass, and not wood. If wood, than other issues issues apply.
The round chines on the GB lead to a very rolly boat. We had ours in Hawaiian waters, and didn't realize it was a PROBLEM . . . It was just what it did, and we were used to it. Now I know about different hull designs, and that that there are better designs out there.
For us, the interior layout of the GB's is . . . . well, the only GB I would even consider buying now, because of the interior design would be a GB 49 Classic, but they're pretty rare, as in only about 125 built. There's one in available locally that I'd buy if we didn't have our current boat, but it need some TLC. (Sorry, that doesn't help you in Australia, but they may have some down there.
You mention day trips mostly. . . . No shade on the outside, and not a LOT of room inside in a GB 36. I would prefer something with a Bimini aft to provide shade, but in the Classic, with the trunk cabin, it's just not in the cards. You may be better off with a Europa style, as mentioned prior, since it will give you more usable space.
I would NOT want a GB 36 with twins, unless you have a skinny circus contortionist in the family. And just remember, even if the kids fit now, they will grow . . .as will you, and components will be DIFFICULT to access and maintain with a twin. Our GB 36 had been recently repowered with a Cummins 6BTA. Great engine, WAY more power than the boat needed, but it was already there. Personally, I'd shy away from Volvo's, for the reasons specified above. (Parts cost and availability). Not sure what Volvo parts availability is like in Oz, but we had a dockmate in Washington State who had been waiting for parts for over a year. His (not a GB) was a twin, and he'd been using it a a single for almost two years because he couldn't get the parts. I believe it was a manifold he needed, and he had it on order through at least two sources. And he's had to pay up front for both . . . he figured that if one was bad, he'd better order two, one for the other engine too, and hopefully the 2nd one would last until he got the two in . . . but over a YEAR?!? That's crazy. He considered repowering . . .too $$. He considered selling as is . . . he'd take a bath with a non-running engine he couldn't get parts for. His plan was to get the parts, repair, and sell immediately . . . any more parts issues would be the next guy's problem.
Best of luck in your search!