When you have finished the woodworking, before you get into tuning the cab, you should seal the wood inside and out with several coats of something like polyurethane varnish.

If you cover the outside with Tolex, you will want to add some sturdy corner hardware to reduce wear and tear on the vinyl. A big trunk handle on each side may come in handy too.

For speakers, I suggest you contact Raskolnikov for his recommendations. I know good bass when I hear it, but I don't know a lot about how to make it happen. :)