Another suggestion to offer is go for the best one you can afford to get. Nothing sucks more than you drop $500 now, when you "could" spend $800 on something better but you want to save a few dollars only to find out you really wish you spent the $800.
Unless you have a really top end unit, your not going to get your full money back on a used wdh or even close to it.
That said, you have a fairly short moderately weighing trailer, the reese pro series may be exactly what you need.
I have a pro series with the round bars. Started off with one friction sway, now I have 2 of them. It works pretty well, but I'd like to to be better. I wish I would have just started with a reese dual cam instead or a blue ox. But now thats going to cost me quite a bit, even if I can sell my current one to offset some of the cost.