KB7 said..
I assume you will be doing upwind/downwind off the western reefs of Lord Howe, such a nice place. I wouldn't go bigger than the 870 because you will have proper ocean bumps moving fast. I'm 85kg and use my 870 for light wind winging and parawinging. I sold my 980 as the 870 renders it totally redundant. It gets up the same or easier, glide is next level giving you much more time to pack the parawing away while the foil auto glides and it's much faster.
If I was riding fast ocean bumps I would want the 770 or maybe 670 so the 870 at your weight will be ideal. I ride mine with the 130 surf tail and it turns surprisingly well once you shed the speed a bit.
i'm about 90 kgs and use the MA 990 for foil driving. sometimes i think even that 990 has too much lift for me with the FD. Do you think a 770 UHA would be the next step, or is that too small for FD at my weight? I've tried the 780 HA with FD but couldn't even get on foil with it so worried UHA 770 might have same issue, but think UHA 870 may overlap with MA 990 too much.