I would build it on a concrete slab with room to add/extend it later.

IE: Don't build it where you can't poor more concrete easily, or that it requires you to grade it to add more concrete/building.

I have a pump house that's probably 8x8 (max!) and the slab out front sloaps downard, almost like a driveway it's about 12' long, and it baffles me why the original owners did not move the pump house back 5-10' (TONS of room) and make the slab 100% flat out front so that we could add-on later... oh-well.

I would insulate, and drywall it, or OSB if you can (code?). My pumphouse is not finished and there are spiders, webs, etc, all over... finishing it may cost an additional 200-300$ but IMHO well worth it, and adds tons of value not to mention better temperature control.

If it's near your bedroom don't go witha metal roof or you will here clank clank clank all the time from rain, snow, leaves, etc... Home Depot is clearing out their 30yr comp shingles right now for a killer price, and for a building that big the shingles will be around $75 total $25 paper = $100 to roof.

Go with 2' eves so you can lean stuff under it so it doesn't get wet.

You could add whirly-birds for vents to keep air flowing too, attic or not good idea.

I would def. add electric if it's affordable to run it out there, if not and it gets sun you could add a cheap panel and a CFL or a couple LEDS inside to light it up.

Make sure it's sturdy and has a deadbolt