"If you are having problems finding large enough dowel pins, you can also use metal dowels (aka metal rod)" I would stay away form metal unless you use about th esame diameter (3/4 to 1 inch) as a wood dowel. While the metal wil be way stronger than needed (a 1/4 inch or even 1/2 inch would be strong enough) a small metal insert can easily exceed the crush rating of the wood it is on. This would cause the post to simply split with the smaller metal dowel from load concentration.

There’s actually a website! Let me remember the name, it was in Wisconsin I think. I ordered from there when my oldest went to college, I also ordered a top sheet and flannel blanket that were fitted at the foot, so that the sheets didn’t hang all wonky on the bed. That drove me crazy when we toured colleges and saw the dorm rooms. Edit: bedderbedding.net Very primitive website but the sheets lasted for both kids in college.

I really don't know what people mean by MDF is not structural, MDF must be reinforced or engineered to bear weight and be used as a support structure, but it can be done. My main workshop table is an MDF torsion box and it doesn't sag under far more weight than bunk beds will need to support. I chose MDF specifically for that purpose because it is a more stable surface than anything else. However, the devil is in the details. You can certainly build MDF bunk beds that are strong and don't have any sag, however, there are designs where MDF would not be optimal. It is really hard to discuss without having some idea of the bunk beds. It is likely that a bed builder will add some solid wood posts as it is cheaper and takes less space than doing the same with MDF.

What type of paint would you recommend for built in bunk beds. I want a really tough finish