That’s going to come up nice!! I like seeing these ideas. I guess try get an unfinished one if possible or just a varnish finish since you’re going to paint it. There is a chance with old painted ones that the paint may contain lead. Newer varnished ones would be safer. You’d likely have to seal it with a primer/undercoat then your finish coat. I wouldn’t think the paint would be a big issue on the outside – unless Turbo is a chewer when he’s out running around. Many paints now are less toxic and eco-friendly.
For the inside, it may be already be left unfinished which would be ideal. What you could do is sandwich the wire between the armoire wood and some lengths of untreated pine. Does that make sense? The pine would line all the wood on the inside. Then the only wood inside the habitat that is accessible for chewing is a safe wood.
If you can source an unfnished pine armoire, then you’re way ahead! Maybe a furniture making project somebody started and never finished. Most woods are ok, avoid red woods which are toxic if chewed. For the base of the habitat, you could just cover in something suitably safe and water proof. Other things to watch out for with second hand furniture is mold and what has previously been stored in it.
How did you get on with the persian rug? Is Turbo over his fear of it?