The thing I've learnt most, is that goldfish need a lot of space to grow and exercise. What cold water tanks have you kept in the past? Just the small 62 litre?
The guys here have pretty much hit the nail on the head. A bigger tank will be much appreciated by the fish!
For fancy goldfish, the minimum stocking is around 80 litres/20 gallons for the first fish and then count 40 litres/10 gallons for each additional fish there after. So depending on how many fish you'd like to keep, you can work out a rough tank size using the guide
I think tanks also need to be a minimum of at least 4 foot or so, to allow for exercise.
50% weekly water changes is also a good way to go along with having lots of filtration, generally 10x the tank size is advised, so for example in a 150 litre tank, you'd want around 1500 litres per hour being turned over.
If you shop around for external filters (these have loads more room for media than internals), you should be able to find a good deal
I know a company in the UK (All Ponds Solution) which sell reasonably priced filters -I currently use their 1400+ model- and I'm pretty sure the filters are available elsewhere under different names (SunSun, Eagles Aquatic, Aquatop).