Access to free panels sounds like "used panels". Make sure that they are of the same or close wattage and voltage, because MPPT works the best when panels are closely matched. It will still work with mismatched array, but not as efficient.
Also, panels are aging at the rate ~1% a year, so 10 years old array of nominal 500W will work as 450W. So plan accordingly. Panels with glass too scratched will have lower yet efficiency, so pick only those that look good.
Panels brand - I prefer known brands in both panels and controllers, but with free panels it doesn't matter much.
MPPT controller type and brand:
Make sure controller has enough input voltage, this is particularly important when running a series array of panels. In parallel array only amps are added on the input, while input volts are the same as of a single panel.
Nominal controller output in amps is less important than input voltage limit, but still important. MPPT won't fry if total output amps exceed its rating - it will just clip it off, so you will lose some power at the peak sun. To give you an idea, for 500W array you need 30A controller in most places, and 35A for high elevations. I would suggest Rogue 3048 for array up to 490-500W. For 750W it would be 45A controller (or 60A for high elevations), and in this case I would choose Morningstar 45 or 60 respectively.
I would avoid no-name MPPT controllers from Ebay. Their efficiency is unknown, reliability is questionable, they are often lacking essential features like adjustable setpoints and remote voltage and temperature sensing, and the very MPPT feature can be a fake, i.e. they simply cut down the volts from 24 to 12 without proportional increase in amps. Also, the may have a low limit of input voltage.
Again on the input voltage limit:
It is important if you connect panels in series, and not important when in parallel. If there is any shading - from one panel to another, or from superstructures on the roof, or from trees - you'll be better off with a parallel array. Even on a good panel with many diodes, total solar production of a multipanel array may drop as much as 50% when only one of several panels in series is shaded. Parallel array is more difficult to wire, though.
