Since it's a guest bath, you might do something that's less utility-ish. In our bathroom/laundry room we made a "vanity" out of Ikea Sektion cabinets and installed a 24" Domjo apron front sink. It looks nice and serves both functions well.

They don't sell domsjos anymore :-(

Do you know what the distance is from the door molding to the back of the alcove? We should make sure you have enough depth for using kitchen cabs.

I don't know much about the domsjo replacements, Havsen. I plugged them into the planner and it looks like the 21" x 19" drop in havsen plus cabinet, but not including counter or faucet is around $290 with a veddinge door (that's what we have and although it's a less expensive Ikea door, it's held up pretty well over three years. )

The apron front havsen sink plus cabinet and veddinge door is around $350.

We grabbed a section of preformed laminate countertop in the Ikea as-is section to help keep costs down a bit.

below is the drop in havsen:

Apron front havsen: