What fish could i have? (planning on a few goldies)
Koi, goldfish, tench, orfe etc are all popular pond fish choices. The type of fish you can have in your pond depends mostly on the size and depth and what the climate is like where you live in the world. I would strongly advise against stergeon though, as they grow massive (the smallest types available still grow to 1.5-2metre's long) and they are really not suited to pond life at all, needing near freezing water temps all year round and very highly oxygenated and filtered water.
Koi need a pond of at least 4ft depth, while goldfish need at least a 3ft deep pond. Ponds of less than 3ft depth are not really good for anything other than to serve the purpose of a non-fish wildlife only pond- having a deep enough pond is very important for the fish involved, the deep depth keeps the temp in the pond more stable and allows the fish a peaceful hibernation during the winter and also helps give the fish protection from predators. If the pond is too shallow for the fish and the pond freezes over during teh winter, the fish can die from freezing.
I would only advise a pond of less than 3ft depth for various pond fish if you can afford a heater for the pond to prevent it from freezing over during the winter or temps falling below 10degrees.
Are pre formed ponds good?
There aren't any benefets that i can think of about pre-formed ponds over ones made of liner and stuff etc. Pre-formed ponds can be quite difficult to fit into the ground as the hole dug for them needs to be quite exact to their shape. Pre-formed ponds are quite sturdy, but if you build the pond properly then one made out of liner and underlay can be just as sturdy.
Does it need to be filtered?
If you are going for fish then i would say "yes". Goldfish can live in unfiltered ponds if the pond is very lightly stocked with fish i.e. quite understocked (i would say about 1 third stocked), but adding filtration still offers the fish a much better quality of health/life in the long run and will also help keep the pond mcuh cleaner, which is very important for fish like koi, tench, orfe, goldfish etc.