BZ has a 250 input-watt MPPT controller, the MPPT 250 model, that goes for around $150. (I've never used one of their controllers, so perhaps someone else can reply with some personal experience.) Since your targetted capacity of 20A is pretty close to the maximum for the MPPT 250, something to consider is whether you'll likely add more panels in the future. If so, paying around an extra $50 to get the MPPT 500 model (500 watts input) might be reasonable if it keeps you from having to upgrade the charge controller later on.