Hi there all, I've recently bought a new place and the back of the house has had a cheap renovation probably 15 years ago to extend a decking and outdoor laundry toilet on one side of the house. On the side where the laundry and toilet are, the external facing wall is falling apart. It's a stud wall with cement sheeting and render on the outside. The stud wall was constructed with 90 x 45mm (untreated) structural pine with 600mm spacing between the studs and doesn't appear to be load bearing but is supporting 2 windows. There was no sheathing applied to the stud wall, the cement sheeting was applied directly to the studs with nails and then rendered and painted. The sheeting is now falling off as the stud wall is rotted right through in some places, I can crumble away pieces of the bottom plate almost entirely, and studs and noggins are rotten in many places also. If I had the money now I would tear down the whole renovation and rebuild but at this point I just want to repair it buying some time until I decide what to do long term. If I replace the wall, I'd like to get peoples thoughts on the following:

1. Approach - Start by replacing the bottom plate (using props if required for the windows); then working from left to right remove existing noggins, insert new studs at 450 intervals (as opposed to the existing 600) and add new noggins; replace any rotten pieces of wood on the window frames (these are hard wood so in better condition than the frame.
2. Sheathing - before attaching the cement sheeting, what sort of sheathing (if any) should I use? Would plywood suffice? Will this provide enough protection from the elements?
3. House wrap- Do I need to use any for waterproofing? Especially given I'm not repairing for the long term...
4. Timber - if I apply sheathing then I'm going to have a step from the existing brick wall if I use 90 x 45mm for the studs instead of a flush finish. Therefore would 70 x 45mm F5 pine do the job as the wall does not appear to be load bearing and is just 3000m high. I don't want to re-plaster the inside part of the wall, so I can't set the stud wall back to make it flush.
5. Cement sheeting - do I need to use waterproof blueboard cement sheeting, or will 6mm external grade cement sheeting do? Or, can I use blueboard and get away with no sheathing.

Appreciate any help/comments and apologies for the basic questions, I ain't a chipy just a DIY
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