Hi, and welcome to the forums. My advice is to buy the trailer that you really want, that will be worthy of the hours of work and $$ of rennovation expenses you put into it. $500 Airstreams in need of complete rebuild are not that uncommon, don't be lured into buying it just because the price seems like a steal.
One of those pictures looks like the spot where the water heater should be installed. The subfloor is rotted through there, and you can even see weeds growing up through it, suggesting that either the belly pan is missing or in really bad shape. If there is that much rot under the water heater, then chances are the whole rear end is rotted out as well. If the floor is that rotted, then the frame is probably disintegrating also. So what you get for $500 is a trailer that will take another $2000 and a year of labor to get the floor and frame solid. If this isn't how you planned to spend your time, then you should pass on this trailer.
You won't know how bad it is until you really probe around. Do a search on the forums for "buying checklist," and you should get two or three lists that enumerate all of the things to look for. In your case, you aren't trying to talk the seller down on price, you are just trying to understand the scope of the repairs that will be needed to make it safe.