I have a shed that was build with lath and plaster in the back yard. Some of the wood on the corners rotted, and have exposed a few holes. I planted creeping fig, in hopes it will engulf the shed and build a bio structure.
Take a good long look at the shed and decide if it's worth keeping. Also check the structure, usually you can salvage the skeleton and rebuild the rest. You've already got the slab, that is good. Fix the rotting parts, repaint it, seal it, etc. Go cheap, go expensive, it's your choice. Consider using reclaimed wood from your local habitat for humanity. Same for roof tile, did you know that mixed roof tile looks kind of cool? Go to to a builders or doit yourself type of forum, etc. For roof, do you like the sound of rain hitting metal?, or rain running through a gutter into a water reclamation tub or barrel. The sky is the limit with these backyard sheds, just make sure your local town/city knows whats your doing or some neighbor might complain, etc.
How close are you to the ocean? The reason why I ask is because salt air can corrode. If you want to make it for livable space I'd recommend cedar over pressure treated wood. Mostly because the old ACQ stuff had material that was found to be I think cancerous. Granted they changed it but some get apprehensive with it. For metals personally I don't like galvanized...stainless holds up better and doesn't get the hands that much dirty..it also can look a bit better but you are looking at nearly 10/lbs.
Not sure what your question is. Yes I have experience in that I am a licensed general contractor (semi retired and not looking for work at the moment) and last year built a fancy shed in Marin county (CA). In Sonoma and Marin you can build a detached shed of less then 120 sq ft without a permit, which this was though it was illegal in the sense that it did not have proper set backs from the neighbors property.
I would determine what you want to do and get a contractor to give you a price. Repair work, depending on the extent of damage, is something you maybe can do yourself, running an electrical service and or plumbing probably not. When you say lath and plaster I assume you mean it has a stucco exterior and that is something you can't do your self unless you don't care what it looks like. It could be termite damage or it could be water damage or both.
Does the shed have a stem wall or are the walls sitting on the same level as the slab? Technically, any wood surface should be like 6 inches off the ground. So, since it doesn't have electric or plumbing i would actually lift the whole shed off the slab and put some blocks underneath. We moved our shed by hand with a car jack and round pvc pipes to roll it on and 2x4's for levers... Lath and stucco is heavier, but if you plan to use it for a studio, you need to get it off the slab and onto some sort of curb.... As long as you repair the rot and restucco and paint you should be waterproof.