If affordability is your main concern keep an eye on the buy/sell section and pick up something rated for at least 400gal.
My current system holds about 240gal and has a relatively high bioload as I feed a lot and often because of the types of fish that I keep. For it I was able to buy an old model Precision Marine Bullet 3 used for a small fraction of what they retail for. I'm certain it could handle more than what I'm using it for so I intend on continuing to use it when I eventually upgrade to a larger system (370gal hopefully).
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