If a nicer pre-owned Bennington, then completing some sort of stereo system might be worthwhile.
Likewise though, if the plan is to flip it anytime in the next few years for a nicer/newer boat, then I wonder if other options might be more worthwhile. For instance, adding a decent system on your current boat may not really improve the value of it anyway. For the same amount, or less than your stated budget, you could likely get 1-2 very high quality Bluetooth speaker systems that can sync together. This way you have a system independent of your boat for other uses, one that you could keep beyond a future sale of your current boat, and yet one that meets your near term boat music needs before the future boat upgrade. It also wouldn’t necessitate adding and wiring in a dual battery system if the only reason would be to have power to crank tunes when beached, anchored, or drifting. Anyway, just tossing this out as something possibly worthy of consideration.