You could, though I wouldn't recommend it, as the model is probably too high poly to work well, and you'd still need to do 50% of the work, in the Jbeam, which even for an expert takes a couple of months. I'd recommend your first mod should at least be with a free model, you don't want to spend $150 and then lose motivation.
Depends on the license, you would have to read it carefully, in most cases turning a paid model into a free mod is not allowed because anyone could just take the model from the mod for free
no that's what 3D models of this quality go for Think about it; you won't sell much of them compared to for example a mod or a video game so the price needs to be higher to compensate
