I'd do a project like this over 2 weekends. Purge the first weekend then you will know exactly what is left and work out what you need for storage. Sometimes when you buy storage first you find afterwards it isn't suitable for your needs in the end. Certainly the ideas already suggested are great. Don't forget to allow for future storage as well.

Aren't Koi just fancy coloured Carp? Would eat anything, including native frogs who will find your pond a great habitat. How about a pond liner and run one of your down pipes to the pond with maybe supplementary hose water in dry season? Guess also depends where you live in Oz. Also, yes, evaporation can be huge factor, especially southern states. Cheers.

Sorry, I don't know how to load a photo, but the finished tiles look just like the Ladbroke Trc pavers above only it was done with poured, coloured concrete and then cut and grouted to make it look like pavers, hence no weeds ever! We had it done a terracotta colour which looks great. The sides were done as a wooden retaining wall with stairs up to the higher level of lawn, also done in the concrete. The advantage of concrete opposed to decking is that you never have to think about again as it doesn't rot like decking does after a few years and have to be replaced. It make cost a little more initially, but long term it's really worth the money. We finished off the top of the retaining wall with buxus hedging, but you could put flowers or whatever you fancied there. The concrete trucks have long hoses which can pump the concrete a long distance to the back of your house so no worries there. Hope that helps. Good luck.

Well I know you want to be true to the house & reuse what you can but I would probably go with a more modern style like the stainless wire and timber top rail rather than dowels or timber for the infill. Be mindful of building code for the infill as horizontal is climbable and has to be very close together. There is so much upkeep in timber & if you go with the vertical metal rod look with the timber top plate maybe...? I am not an expert on any era or style though I know you will have a preference in mind & I would go with that. Look at the big picture of how your house will look with a change of paint (are you painting the house?) & for me it was about making the 25 year old house more contemporary. We love it!