Most finishing mowers will have belt drive to the spindles that the blades are fitted to, so I wouldn't be worried about belt drive. That thing is just an offset slasher, so if that's what you want, the price is not bad. It's not a flail mower, but a flail requires a lot more power to drive than a slasher. From what you've written before, I think a simple slasher, or if your ground is smooth enough, a finishing mower.
As an example of what's out there, I got one of
these for the same money as your gumtree example. Secondhand and needing new bearings throughout
Bear in mind the advertisement is American. Aussie prices for these mowers is much more.
Don.