I would say steel,

Yes , I have had two steel boats up till now and I am building my 3rd.

the downsides having to lift it out every 1-2 years to sandblast,

You dont need to blast that often if you look after the boat .I repainted my
hulls only once and only because I wanted to change the colour.

second hand needs exspensive tests to see if any damage to the hull.

True, But a good surveyor can tell if its worth shelling out for the
expensive tests.

The heat can be delt with have good insulation and ventalation.I sailed up
the coast from Brisbane to Darwin in the summer of '98. And then up to the
Solomon Islands for 4 months and my boat was pretty cool inside , 30 mm of
spray on foam from the waterline up and accross the deckhead,5mm air space
and then the ceiling keeps everything livable.

Neil C