Paul,
Ive kept koi for over 20 years and they are a very satisfying fish to keep. I am presently collapsing my pond as I feel that they deserve more time than I have been able to give them over the last couple of years.
I've got a formal (above ground)pond of 2500 gallons including the filters. I have kept mainly Japanese koi that I used to import with a friend but now only have 6 left after selling the other 25 or so that I used to have. One of them, a matsuba was 26lbs in weight when I sold it last summer.
As a result of collapsing my pond, when I have managed to find a home for the remaining 6 fish, I will have some equipment at very reasonable rates that may be of some use to you, filters, pumps, UV's etc.
Personally, I would recommend that you cast a concrete base and then block build up out of the ground and above ground level add an outside "skin" of brickwork. This can then be rendered and then painted with a fibreglass type paint. The gravity filter can either be fed by a bottom drain or by a 4" pipe extending to the bottom, if you have a slight slope on the bottom of the pond this would serve your purpose without the hassle of a bottom drain.
My pond built in this way just required hoovering fornightly (through the warmer months) to keep the water crystal clear. If you do need any advice at all feel free to give me a ring and I'll help in any way I can.
Al
As a post script, I have 6 koi for sale all between 12 and 18 inches, a sanke, a kohaku, an ochiba, a goshiki, a doitsu goshiki and one non descript fish that I have had for 20 years. that all need a good home! Not looking for a fortune, about 50 quid each or so.