Dunno if you realise, but Common and Comet goldfish require about 40 - 55 litres each as they grow, and fancies about 30 - 35 litres each. Goldfish eat more than they need but the beauty is their digestive systems only make use of what they actually need, however it can be lethal in small tanks because they excrete/urinate excessively - its the Urine (Ammonia - hence you have to keep an eye on Ammonia levels in any fishtank, but moreso a Goldfish tank) that is the killer. Thy also require their own space, and will become stressed in a crowded tank, and stress in goldfish, causes infections and sickness.
So you could be looking at 200+ litres for them. Depending on the size of the goldfish a 2 foot may be okay if it holds about 65+ litres of water, in the short-term. It's a common misconception that Goldfish don't require big tanks, but the reality is exactly the opposite.
You can sometimes pick up 4 foot tanks with filters etc for less than £25 on eBay, also check out Carr Farm down on the Meols Stretch, they sell second hand tanks and often have 2 - 4 litre tanke for £16 - £30, which is as good as you will get.
For a 100+ litre tank, you will be needing an external filter, which will cost £30 upwards brand new, but will be the best solution (both in terms of filtration quality and ease of cleaning) and "should" last you the lifetime of the fish, look for ones from good brands like Fluval, Hagen, Tetra, Eheim, Aquel etc, and ones that include a 5 year guarantee for peace of mind.
Dont buy a fish tank brand new, unless you want summit special like a Juwel or summit.
Do you have the goldfish already? If not, then id recommend limiting it to 2 maybe 3 if your not planning on a very big tank.
The bigger the tank, the better, much less hassle as you don't have to worry or keep an eye on them so much. It also means less cleaning and preiodic part-water changes.
Don't forget, there is plenty of coldwater breeds out there, so if you want an active tank, you can get Danio's, Guppies, Minnows etc, which are all good community fish don't require anything like the capacity goldfish require, so if you don't have the goldfish already, you could buy two goldifsh and get other smaller coldwater breeds. Many of these breeds have better digestive systems and both types of intestine, which leads to less urine or excrement.
It may sound like a lot on money for some stupid goldfish, but it should prove to a be a one-time expense, and will ensure your goldies live the happiest and healthiest lives possible.