[Ozric Jonathan]

I agree with thick plywood. My large bunk bed setup is made from thick plywood with decent pine used for the corner posts etc.

For the bases I fixed lino tiles down on top of the ply which was a lot of work and I wouldn't do that again. Sheet lino would have been so much easier.

As already said mdf has bad stuff like formaldehyde in it. I think ply has glue on it as well but not to the same extent. Also the thick ply is really tough and isn't going to break up so the chemicals don't leak out.