Welcome to the Bunnings Workshop community. It's fabulous to have you join us, and thanks for sharing your question about building a low-level deck.
I suggest having quick look at these guides - How to build a low-level deck by @Adam_W and How to build a deck by @MitchellMc. These guides I've mentioned should give you a very good idea of what you'll need to consider when planning your decking frame. To give us a better idea of the location, would it be possible for you to post a few photos of the entire deck? This will give our members a chance to assess the area. We can then make recommendations on what to watch out for when planning your decking frame.
The answer to your questions will depend on the size of the timber you'll be using. Before taking your current decking apart, I suggest taking some of the decking panels off and doing some investigation on how it was built, the current spacing of the timber joists and the size of timber used for the framing. My number one tip for this project is to draw your plan on paper, this will give you a very solid idea of what needs to be done and the materials you'll need to get.