Davo,
The 580 has a good deal more room in front of the console, and more storage capacity than the 520. But I guess you are always limited with storage on a centre console boat.
I run a 60HP 4 stroke on my 580, with a (smaller than factory) stainless prop (cant remember the exact specs, but will check if you need). 3 up, full of fuel (115L) and gear, it will rev to 5700 rpm. Cruising at 4000-4500 revs it delivers fuel economy you wouldnt believe. My friend has a 90 HP 2 stroke on the back of his 520 and the fuel figures along with the noise levels are like chalk and cheese. We both bought second hand though, so he didn't really have the choice of power at the time.
I couldnt recommend a 4stroke highly enough for your application, though having run these mid sized yammies on my last 2 boats I cant really give you a real opinion on a 2 stroke, I'm sure someone here can though. In my view 70HP would be PLENTY, but I reckon a 50-60HP 4 stroke would be ideal (as long as it is propped correctly).
The question of how far out is comfortable is a difficult one to answer. That depends largely on your experience at sea, and in the the vessel concerned. I am happy at Browns Mtn which is over 20NM from Sydney Heads in my 580, provided the weather/ sea conditions are favourable and I am always logged with Coastal Patrol. My vessel is in Survey so has a little extra floatation, but the biggest asset of the UB series is the self draining deck. If you take a green one over the side it will empty soon enough PROVIDED YOU HAVE THE SPEED TO MANOUVRE FORWARD.
Anything over 15knt and the sea starts to get a bit choppy, anything over 15knt and you WILL get WET in a UB.... nothing a raincoat wont stop though!
As far as versatility goes, you are right..... they draw only a tiny amy of water, about 8" in the case of the 580, so are as comfy well upstream as they are offshore.
Thats my (biased) view, PM me if you want some more info, happy to share.
Cheers,
Steve