Well if you have the skills, and plan to make this a quality piece of furniture that you can rattle her sexual cage with. I recommend you do some smaller projects like a spanking bench, as well as a cross. Experiment with the joints. A bed is a major project, and I would probably go with tongue and groove combined with some wooden guide pins, some biscuits at the joints, who knows there are so many possibilities. Also, you can get a wood planer for not too much money (under 200). If you plan on making this a piece of furniture that is both functional as well as visually appealing. Putting the money into it will be well worth it in the end. As far as treated or not. I wouldn't use treated wood unless your going to have it exposed to the elements. If your concerned about keeping it looking good over time, stain and then varnish it for a good finish. Keep in mind, you will need to do repeated staining sanding and varnishing to bring a good long lasting durable shine.

Unless your very good at welding and metal working, I would avoid doing your project with steel or any other metal. You could of course find yourself someone who is good at this type of skill, and get your slave to give him a complementary blow job on the finished product