Well, I bought (10) hundred watt Grape Solar 12 volt panels from Home Depot for $89 each with free shipping, only because the price was right. But only then did I get serious about solar controllers. With an MPPT unit you can hook up the panels in series, much easier than wiring panels in parallel which you generally need to do with a PWM controller, and the MPPT controller can harvest more power from the panels for battery charging.
Then there is the battery bank. 12 volt, 24 volt, 48 volt? A higher voltage bank has a definite increase in system efficiency.
Go to the MorningStar string calculator: String Calculator Â» Morningstar Corporation
, and play around with it. Select panels that fit your bus build, and see what works. A quality MPPT controller costs big money, so you want to buy as few of those as you have to. The cheap ones don't seem to last all that long.