What are the benefits of a factory reset before upgrading to Lollipop if the phone is brand new untouched?
I'd say mainly peace of mind. At this point in time, I've heard it's not necessary to factory reset a new device as it used to be rule number 1 on previous and older devices.
I did it to my moto x when I got it, though I don't really have proof that really did anything. All I can say (to myself mainly) is that I for sure started from scratch.
For the record, I started on KitKat and am still on it since I got phone in Oct. I have the update but have not yet applied it. I'm leaning toward upgrading and then factory resetting it. My main con to that is setting everything back up again (gmail accounts, app settings, wallpaper, launcher, icon theme, widgets, you get the pic
In your case however, I would do exactly like you mentioned. Right after you get phone, upgrade it and then factory reset it, even if the upgrade itself is still going to fully clear/reset those registers correctly, a factory reset can't hurt anyways, especially if you haven't set phone up yet.
Also, factory reset is not a long process, just the setting phone up part.
Hope that helps.