I looked this thread up and down, but don't see what year and model the trailer is that you are considering. This would help.
Just to throw in my 2 cents and share my experience, I bought a trailer from the Florida panhandle almost a year ago. I looked for 2 years trying to find one that would need relatively cosmetic repairs, not a full rebuild. At the moment, the shell is sitting on the driveway, and I have been doing frame work since Christmas. I am just getting ready to start reassembly. Like your prospect, my trailer showed only surface rust on the tongue, but when I peeled back the belly pan, the crusty, scaly heavily pitted frame members became visible. I ended up replacing approximatly the last 4 feet of the frame, repaired about half of the outriggers, and replaced approximately 1' of frame rail in the front. I would expect that your trailer will be at least as bad as mine.
The painted section of skin sounds scary too. My bet would be that there is major damage there, and someone filled the damage with "bondo," then painted it to hide the sacrilege. You can tell just by flexing the skin--the bondo will likely crack, or at least not flex.
There are several threads that describe things to look for during an inspection, and guides for pricing. This trailer sounds like a "field find," in which case you may be overpaying, even at $2500. The floors are probably rotten, mice and other nasties have probably taken up residence in the walls, and the appliances/air conditioner will all likely need repair/replacement.
So manage your expectations. Fixing up a trailer, even one with a sold frame, can get expensive fast. Seems like $1000 is always the magic number. Need a new axle? New refrigerator? New AC unit?