Odd there arent rollers on the bottom of it....

What's the deal with the brakes? They work?

500 might be a good starting point. I would
point out that the trailer might have to be worked
to accomidate your needs.

Take a look at the trailer tires? Cracked on the outer walls?
You need to pull the rims/tire off to take a look at the brakes.
If this is a salt water trailer, that gets nasty unless it was
flushed regularly.