a 500g pond is most likely not the way to get into salt for the first time. that seems like trying to walk before you know how to crawl. the chances for loss of money are substantial if you make a mistake on a system that large.
aside form that, the initial investment would be very high for everything you would need to properly maintian a 500g. salt is a fry cry from freshwater, where you just add a canister filter on, and go.
start with something in the 75-125g range first IMO. its not so small that any minute change could be detrimental, but its not so big as for you to be possibly spending thousands on equipment and stock only for it to crash and burn while your getting over the learning curve and figuring out all the ins and outs of salt.