Cambourne,if hubby wants to keep large goldfish or koi carp there is no preformed pond big enough.They need a depth of at least 4 foot,a very large surface area and expensive filtration.This will all cost a great deal of money and take up a lot of space!!
You could compromise and start small with small,hardy goldfish and then if you get the biggest preformed pond you can fit in,you could still have a pretty addition to the lovely garden you have.