Basically, unless your LFS has some sort of quarantine system (and even if it does) you should assume EVERY fish at the LFS is carrying ich. If you want to keep it out of your display tank, you will need a dedicated QT tank, and QT every single fish. If you dont follow that process, even with just a single fish, you can assume that ich has been reintroduced to the tank
That being said, healthy fish should have no issues with ich, even if it is present in the system. So with wise livestock choices and good maintenance habits it shouldnt really be an issue