Can't help with your listed a/c types but
How important would pressurisation (or lack of) be in this scenario?
depends on your / your passengers' tolerance for wearing cannulas.
IME, I have always managed fine with cannulas, and I've been to FL200 to remain VMC on top.
I've not had to turn back yet, so this approach seems to work.
The caveat is that I generally avoid flying through fronts unless I can check its likely tops and they are low enough to overfly - a policy some will strongly disagree with
Obviously frontal weather can have much higher tops so you either accept the conditions and press on in IMC (with de-ice available), or you need something with a ~ 25k ceiling (which will clear most warm front weather) but oxygen gets very tedious up there so pressurisation gets highly desirable.
However I have rarely done long trips (> 500nm) below FL120, and most were done at FL140-160 to remain above the wx, so that means oxygen on nearly every flight, so if you are really talking about carrying 4 people, you are looking at the inside of the plane being plumbed up like an operating theatre, and no matter how big your o2 cylinder is, it isn't going to last very long, and with refills being a real bummer, you should be seriously looking at pressurisation.
OTOH much depends on your desired despatch rate. If you are happy with say 75% (and usually departing the next day on the other 25%) then you don't need a "flying tank" to do that. Even my TB20 will do it. If you really want say a 99% despatch rate, and your passengers are "averagely robust"
then you need to think more carefully, IMHO. And it will cost you a lot more.