Yes, looking at a purchasing a used truck and being told to weigh the truck isn't very practical advice.
When looking at a used dually, look at the rear tire's printed ratings.
There are two ratings printed on a dually tire.
One rating states that it is for use on a dually (it's a lower rating).
The other rating states a higher rating (for SRW axle).
Take the lower printed rating on the rear tire - double that number and you will be very, very close to the official payload capacity rating.
I've stated this before in another post.
It is a dumb simple accurate method to find payload capacity (unless the tires are not OEM rated tires).
You need a quick and dirty method of determing payload.
You're looking at used trucks - and you'll need to kiss a lot of pigs before you find that prince.
The rig you want, at your price point, is out there. It will take a lot of looking and patience.
Or a little bit of luck.