The ONLY time I would bother with pontoon boats would be if i HAVE to fish weather windows less than ideal offshore and I would only buy an Osprey for this due to their supreme entry and much greater deadrise.
Otherwise Id go with heavy plate, I was a fan of extremes, but White Pointer or a Southern XP. Out of those 3 I would choose. Id also go twin engine if you are into offshore. Or at minimum a suitable size Auxilary.
If I had the dosh for new, and was to be just 6.5m, I would go Southern, If I had more money I would go white pointer.
I totally disagree re Extreme not handling chop, wtflap
Extreme and surtees have a near identical design, the Extreme is heavier plate, they have designed their cabins to create less noise which is the plague of tinnies.
If going WP i personally like the inboards on there 8m plus boats. Diesel for me and an auxilary is a good option.
For you Id probably go a Souther 646XP or 676, I would if possible go enclosed cockpit (dont think you can) but definitely Cabin. Get cork flooring, takes so much noise out of the boat.
So many choices, all are good, nothing wrong with a Surtees either, probably save you some money.
You do get the safety of the water ballast in the surtees and extreme, they can be known to porpoise but just trim it.
The WP does the best Enclosed bifold cockpit imo, with a toilet and shower building totally seperate outside, so if doing 3-5 day trips is worth it.
Many boats offer pull down lockable cabins making them private if its you and your Mrs and your mates outside in the cockpit etc.
So many options, you are asking how long is a piece of string hehe, WA Aus makes some great plate allies, Melbourne is known for the best Glass boats in Australsia, but NZ does produce extremely good plate ally boats.
Of the pontoons, the Osprey 650 tickles my fancy. I have been on the Otago Uni Marine Biology boat, Tuhura. They have chosen this boat for the southern ocean for good reason.
https://www.otago.ac.nz/otagobulletin/news/otago656616.htmlPoint is you are going to have to boat test any boat that interests you, so pick a few, go for a ride, take note of what they say suits the hull, go from there.
Best wishes !
Kia Kaha