I hot-dip galvanized the frame on my 76 Bronco during my resto-mod...a buddy from HS was a saleman for a plant down in Nashville, TN, so I brought it down to his plant after I did all the mods to the frame. The hot-dip process is, if I remember correctly @ ~800 degrees, so thin sheet metal is not typically dipped as it will warp badly. I am not sure if the tub of an M416 would hold up well to the dip, plus there would be alot of excess slag on it from cooling as it came back out of the molten zinc bath. It is possible that they dipped the frame while it was all bolted together...even though its all pretty thin metal as well. That would be pretty evident by looking at the bolts. A close up photo of the frame would tell alot.
That said, there are various grades of cold-galvanizing sprays...from cheapo to really good products. If they were used here, thats not a terrible thing; the key here would be preparation. As well, to get a nice coat of cold-galv on that much surface area is still pretty freaking expensive. I would guess $200-300+ in cans. It will scratch, but can always be easily re-sprayed. It all depends on what you like. The pic looks like it was sprayed to me...but if it was dipped...and that may be possible...it may have been done a couple of years ago and thats why it has that look to it...but to me it looks alot like the galvanized-primers that they call "Weld-Thru" primer.
When a frame is first dipped it almost looks like chrome, it oxidizes fairly quickly over the next few months to have that "galv" look to it...top-coating is always do-able, but you have to use the right products and follow a sweep-blast procedure to make sure that it will stick....and this needs to be done shortly after being dipped before the zinc can oxidize much.
For what its worth, I think they trailer looks prettty sweet and seems to be outfitted well. If its in your budget and you like it, go for it. As another poster mentioned, you could very likely spend that much or more and countless hours of time setting up one similarly, so its more about whether you want a project or you want a useable, ready to go trailer...
Good luck!
John