Hi Captain,
At 85kgs and typical Cook Islands conditions Id say you'd be pretty safe with a 10 & a 12m & a twin Tip Board around 138-142. There's some great secondhand gear to be had at the moment. I always look for the latest model gear as this is where you are gonna get the best value for money. If it's more than one season old the risk of getting faulty/damaged/worn components can increase significantly in my experience. There's plenty of gear that's only been used a couple of times and in 'as new' condition. You can save extra on older stuff but there's a much greater chance you'll be disappointed and out of pocket if it fails.
Hope this helps, have fun up there!