Just call him and get his opinion. My advice? Go as big as you can because you'll outgrow it!
Lowes has a pretty decent safe for 3-400. 2 people and a dolly could get it upstairs no problem. Big enough to hold a few guns and ammo.
Seems like I saw a flyer in the mail this week where Tractor Supply or Orscheln's had Fat Boys on sale at 15% off.
A safe is like insurance...the better the one you can buy, the better your protection. Or the more valuable the stuff is you're protecting, the better you should consider protecting.
Something to keep in mind - I don't know how they figure how many guns will fit in a safe, but it's typically HIGHLY inaccurate on the low side. I don't know what 12 long guns would fit in that one, but they must not be very big or have any optics on them..