hello there, I would suggest if your going to use wood that i would completely avoid MDF or any other particle or strand boards. I've used MDF a number of times on multiple aquarium stands and while it can be made water resistant it can never be made completely water proof, the smallest amount of moisture whether it be direct contact or high humidity levels can cause the board to swell and fall apart rather easily. My suggestion for your project depending on how in depth your getting, i.e. water feature and such, would be lauan plywood, it comes both one side or both sides sanded giving you a nice finish to start with and its stronger and more durable than MDF. As far as toxins go i wouldn't be too concerned with any contained in the wood itself, although do avoid any that contain additives to prevent rot, such as green treated wood, as you are going to want to seal the wood to prevent any damage from moisture and to simply make it easier to clean.
A couple methods I've seen for this would be a simple varathane or polyuerathane coating such as used for finishing tables, floors, etc.. This is a simple cheap method although from personal experience I try to avoid it since it does not give you a true 100% water proof seal and it tends to wear away over time with cleaning and getting scratched in an enviroment such as a vivarium. If you do use this method i would reccomend applying 6-7 decent coats with a very light sanding using a 200-400 grit sand paper in between, also remember to clean any dust between coats using a clean cloth.
Another method i've used before in the contruction of salt water aquariums would be an epoxy paint, This stuff will provide you an incredibly tough and completely water tight seal. You can buy it in a multitude of colors, but generally this isnt going to be a product you can run out to your local hardware store and buy. There are a number of different brands and uses it is made for but generally if it is advertised as safe for potable water it should work. If you have any local pool supply stores or landscape stores that specialialize in aquatic gardens you may be able to find it there as it is also sold as pool and pond paint. You can find out more by doing an internet search of "non-toxic epoxy coatings".
Anyway sorry for the rather long reply, I hope i was able to answer your question. If you have any more please feel free to ask.
