If it's free, why not!
Only thing I can think of on the pump, is you might have to clean it a bit more than you would if it were a swimming pool.
Pump's don't like fish poo, but I doubt you will be pulling much in from a pond anyway.
You could make a nice fountain out of it for aeration.
Not sure what to do with the sand filter tho. Just not a big fan of sand filters like this.
Someone else should have more info on how to utilize your sand filter for this application, even your pump for that matter.
Free is good!
