We used Bunnings flat pack shelving in one of our walk in wardrobes, I'm pretty sure it is Melamine and it is fine. There have been a lot of advances and improvements to these products over the years and they aren't the cheap nasties they used to be. Ours are a matte finish too so look much nicer and classier than a gloss. Have you asked the joiner for a sample of just what he is thinking? This should give you a good idea. Check out Bunnings flat pack products too. You could use just an MDF and paint it, but that is more work and the paint can chip or wear off on shelves. Unless you want to go to the expense of having all solid timber, you will find most products are MDF coated with a timber veneer or a product like Melamine. Going on what I have, and I know Melamine was used internally in my new kitchen cupboards, I don't think you will have any problems. If you are having shelving, do ensure you use adjustable shelving too - it is such a bonus in areas like this when your needs can change over the years and it is so easy to just add or subtract shelves or adjust the height. The wire systems are OK for some things but over the years the plastic coating cracks and breaks off and you could then have difficulty replacing the baskets. You are also more restricted with sizes.

A friend of mine used slices of tree trunk, bark attached. The rounds looked fabulous and were placed just right for stepping from one to the other. They may not last forever, however they should be okay for a few years and would look fabulous in an area of trees.

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sawdust plus glue = wood fillers for cabinetmaking so what else can be made from that combination? but are there already products on the market using sawdust?...