Consider using plywood webbed joists. You should be able to price them out at a local lumber yard. They are lighter and stronger than 2x lumber of the same depth. If you are using construction grade lumber like spruce-pine-fir, 2x10 @ 16" spacing is not enough. You don't want a bouncy floor. Also remember that the farther apart the joist are, the thicker your subfloor needs to be.
I asssume hat, for a deask top, you want a very smooth surface regardless of the sheen you choose. I would take a scrap piece and try you finidh of choice with and without using a pore filler and see which results you prefer. I have used aqua coat and find that it really does even out the surface any better than a sponge applied coat of Arm R Seal.
If you decide to use a pore filler, I see nothing wrong with using it only on the top where you want the smooth writing surface.
One option is to use Arm R Seal: Apply to the top with sponge applicator and wipe/off on the rest of the desk.