Spend the minimum on structural changes. However, it sounds like no fence could raise buyer objections and invite discounting. The deck can be added, or not, by the new owners who will want to make their mark on the house. Spend time and energy styling your interior for 'fall in love factor' that speaks to your ideal buyer. This diminishes objections every time!

It seem like you have western sun on the back of the house due to the blinds in you patio photo. To combat this the new roof could be angled low on the west and high over the house as in a fly over roof. Maybe shade sail over the spa attached to the patio roof and lots of tropical plants. It is hard to guess from the photos but if you link the two areas with decking & add gardens surrounding it for shelter. Do you need a fence around the spa? Others have said that the composite decking is good and it won't rot like timber or at least is low maintenance for a wet area.

Not an Australian product, but definitely a lovely alternative to the indonesian woods. Worth a look, http://www.hardwood.com.au/media/roasted-decking/roasted-decking-brochure.pdf

Hi. I agree painting the posts & blocks (if you need to keep them for structural support) white. Another idea if you are handy is to build a timber rail and balustrade (painted white?) in place of the lattice. The lattice does provide some shade lower down, but maybe you want a different look. Your bricks are a good colour, and I like your Acapulco chairs and the macrame pot hanger. I think you could run with those colours (along with the white paint idea) and pull off a Mexican theme in this space.