i prefer my ponds to be elevated at least 6" above ground level. this prevents runnoff from the lawn / garden from washing into the pond when it rains.
as for fish, predation is always a concern. birds, racoons, and possums are all known to raid ponds especialy for big fat slow moving fish. once the racoons find a pond they will keep returning until they are trapped and relocated. there are several devices to discourace wildlife.
as for fish, if you decide to go with somthing smaller, and not a goldfish, i have had alot of success with gouramis. they are colorfull, active, dont eat plants, and will even breed in the pond if the conditions are right.
have fun.