Hi Ben!
The short answer is that the AC50 should be perfect. The long answer follows, and explains why.
A planted tank actually needs less filtration than a non-planted tank, because the plants also help remove wastes from the water.
But unless you're using a tank style where most of the nutrients are in the substrate instead of the water, a planted tank actually needs more flow. This is required to carry the nutrients in the water to the leaves, where they can be absorbed.
A good guideline is to take the number of gallons in your tank, and multiply it by ten. That's how many gallons per hour (GPH) of flow you need.
The AC50 is rated at 270GPH. Just about perfect. Of course, the real-world flow will typically be less, as no one keeps their filter media perfectly clean! But the 10x guideline is just that, only a guideline and not a rule; and you won't have problems if you fall a little short.
There are other possibilities. For example, you could use an AC30, which is rated as sufficient filtration for a 30G non-planted tank. And then add a powerhead to provide the extra needed flow. Might still fit into your under $50 budget. This has the advantage that should the filter's media become clogged enough to seriously restrict flow without you immediately noticing, at least the powerhead will still be providing unimpeded flow.
Sometimes I use that filter/powerhead combo, sometimes just oversized filters. Both work.
