Hello Heidi.I think your best bet if you can make yourself stick to just the 2 fish,is to get the biggest and deepest pre formed pond you can afford the space and money for.Then you will have some digging to do!! but at least the depth should be sufficient for reasonably normal winters.However,you could always bring them indoors for the winter.That is what mum did with hers and they did well for years like that.You could get away without a pump/filter if you have plenty of plants and the pond is in a shady spot so that the water does not overheat.
However,bear in mind that you are likely to get lots of frogs and possibly newts invade also.If the pond is big enough they will be ok,but I have seen frogs completely take over a goldfish pond.