set up a QT tank and QT them for around a month to be totally safe or if you wan't to use parasite meds then you could have them ready in a couple weeks
First buy around 4-5 dozen fish and give them a quality screen try not to add any fish that are really sick or have visible parasites i have a turtle which makes it eazy to get rid of sick goldfish but if you don't have a way to get rid of them then just try to QT them really all you have to do is keep the water clean and remove any dead fish and keep an eye out for any sign of parasites