Why convert? Because used children trailers are sometimes quite cheap whereas dedicated cargo trailers are rarely sold second-hand.
As for "which one to convert"? I won't give you specific brand names, because it depends what you want to do with it and the number of modifications you want to do on it. Here are a couple of points:
- You would be better off with a trailer that has no structural member at the top. My old 2-children Chariot had its main structural member going through the top section; it meant that I could not modify the trailer to open its top. Bending over to put 20-kg bags of stuff is not exactly a pleasant thing to do, and it's impossible to park a bicycle in that trailer, whereas one with an open top can be converted for such use.
- Front and rear door: You'll probably have to modify or reinforce them, so that what you carry doesn't push its way through the door.
- Floor: Usually a bit on the weak side. I would try to line the floor with some Coroplast... which may be available for free soon after some of your Primaries (it's what election signs are made of). You'll probably need some duct tape on the edges.