My biggest concern with this set-up would be rattleing. I wouldn't want any of the wood to viabrate when i turn it up loud.
As for wood, i wouldn't use the cheapest one you can find (we call that plywood). Rather go for something slightly more solid, so you don't have to be concerned about how much weight you can put on the table. I would also consider how it looks, as in pleasing to the eye. Maple, Oak, maybe Walnut, then varnish them to ensure that they will last you a long time, and look beautiful.
If you don't care about looks at all, just use the cheap plywood, but ensure you secure your joints tightly, because if it starts to rattle once you turn your speakers up, you will go insane.
As for the sub-woofer, most studios don't have one. If you buy a pair of monitoring speakers, they don't need, nor reccomend a sub. That said, I actually use a sub in my setup. I bought Tascam VL-21 monitoring speakers (for their size) and when combined with their sub, they suit me nicely.
So if you go for the sub, put it on the wall, where you can see it, but be warned of rattling issues.
good luck
