Ive had 6 boats in 10 years. Im not proud of that, I should have been able to decide on the right boat for me, much earlier. Broadly, I went from big to small and then to in between - and from fibreglass to aluminium to fibreglass; before ending up with a Figlass Firestar (5.3M), which I love and will stay with.
What will be good for me wont be good for everyone. General points I will make -
Its hard to decide on buying a boat, without first having a lot of handson experience. Asking on this forum will help. Asking other boat owners will help. Having a ride or two on others boats would help. Probably end up trying a boat, and later changing to whats more suitable.
I think aluminium is - tougher, ie less susceptible to damage hitting things (like wharves, rocks near shore, shells on the shore, sinkers dropped that chip fibreglass). - and lighter (thats lower towing weight, but harder riding in choppy).
Fibreglass - is susceptible to impact damage, as its brittle. Can be fixed. Also, probably softer riding, if its heavier.
The ride in chop, was crucial to me, as Im 76 and dont like hard ride. Note that in addition to weight, ride in chop is worsened by a hull shape that does not cut through water as well. Consider three things - bigger/longer is softer ride - sharper hull bottom is softer, usually indicated by the deadrise angle (thats at the stern); 16 degrees is low (hard), 21 degrees is high (soft). My firestar is 21 degrees, the main reason I chose it) - and my previous boats were aluminium with pontoons outside the hull, which prevent cutting through the water, and give a harder ride).
I have heard that women prefer fibreglass, as its nicer looking; and also softer riding. Aluminium quickly becomes pretty stained looking (from oxidation); that some dont like. But, if you dont mind the oxidation) aluminium would be easier to maintain.
Dont forget the towing length and weight - legal maximum towing weight seems to be uncertain. THe firestar tow weight is about 1000kg. I would be happy towing with a 2.4L Mitsubishi Outlander. I wouldnt want to tow a 1500kg rig with it.
And finally, manouevring and parking can be a problem, as a boat trailer is longer than an ordinary trailer
You wonder if could take the boat to Great Barrier. I wouldnt do this in my boat. That should be considered with caution. ON a very light wind day, could do it ok. But, even moderate wind causes sea to become rougher than I would like in a 5m+ boat. As its a long way, sea conditions couldnt be relied on. I suggest you forget that idea, at least until you have a lot more experience.