pond advice

[Leliel]

my husband has been talking about digging our pond out and extending it to the width of our garden (about 9ft) and extending it to about 5/6ft in width, with a bridge over the middle (so we can get to the back half of the garden) for some time now, as the pond we have is about 4ft deep (plus about 2ft of sludge by my reckoning) and maye 4ftx5ft. we want to have a japanese themed garden, or an old english garden theme, we cant decide LOL but we do agree that we want silver Koi, and some lilypads. As i do Bonsai for a hobby we're also going to have a weeping-willow bonsai for some shelter on one side, and we would like a water feature of some sort, something to give some air and life to the pond, a waterfall perhaps, but i have no experience of digging ponds! having read some of the threads on this part of the forum i have learnt its not just a case of digging a hole. as the pond is going to take up all but maybe 1/2 to a foot of the garden on either side, would it be best to shore up the sides with something more solid than just dirt to stop the fence posts falling over?we also havent decided what we want to put behind the pond, as we want to screen off part of the garden entirely, entrance only gained by going over the bridge and under an arch, if we go for a japanese theme then we'll use bamboo screening which is lightweight, but if we go for an old english theme we are thinking of building a ruined wall, IE one that has some decorative window arches but only half constructed that we will grow some ivy or something up to make it look like its a ruin, how would this affect the pond's wall strength? would it need foundations? any general advice on keeping Koi would be greatly appreciated too!