Yes i think a pond with liner would be better than a pre-made pond shell; if you built the pond above ground with the container it would be easier, but then again not everyone likes the look of above ground ones (although they can look very good).
When digging a pond with liner, after you've dug the hold you need to plaster the bottom and sides of the pond with wet sand to help smooth out the hole. You then need to get some underlay fabric, it will help insulate the pond against the cold/sudden changes in temp and help protect the liner against stones and things (you should obviously take out any visible stones in the hole you see, but its still important to get some underlay fabric to protect the liner none the less as creatures like worms can shift the position of deep down stones closer to the liner over time which can puncture it etc).
Once you've got the sand and underlay in, you need to measure out the amount of rubber liner you need- try and go for the best quality one you can get, its also always good to get quite a bit more than you need as you certainly don't want to end up having too little pond liner and have to order completely new liner. Its best to have a few people that can help you put the rubber liner in as its actually surprisingly very heavy and difficult to shift stuff.
You also need to decide on what sort of filtration you want, filtration is particularly vital for koi ponds as koi are very heavy waste producers- filtration for goldfish ponds is not essential however if you don't get any filtration it will mean you have to under-stock the pond a great deal and plant a lot of plants to help prevent ammonia levels rising etc. On the whole though, filtration is very beneficial for ponds and pond fish, it'll help keep the pond cleaner, fish healthier, and also you can always get something like a waterfall etc too for the pond which will look great
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Pond dechlorinator is quite cheap stuff but pond plants may set you back a bit if your pond is quite large and you want to give it a nice natural look- pond plants are also beneficial in helping lower nitrate levels in the pond and looking great in general, but most importantly plants like lilies offer fish a lot of protection from predators.
When it comes to making your pond as safe as can be from predators, the more steep you make the sides of the pond the safer it will be from predators- predators like cats and birds like herons prefer to hunt at ponds where there are very shallow area's where they can either enter the pond in or grab fish from easily.
However, ponds which have shallow area's are a lot better for wildlife since the shallow area's will heat up more and quicker in the sun, making them ideal for growing bug larvae and also for amphibians like frogs and newts- however goldfish and koi usually decimate ponds of any wildlife they have in them, goldfish and koi have very gluttonous appetites and will eat almost anything that fits into their ponds which can't escape from the fish. If you do want to hear the croaking of frogs and see the flight and hum of beautiful dragonflies and damselflies etc, then you can always set up a little shallow wildlife pool feature near the pond if you like which would be very easy and quite cheap to do (i am considering doing a little frog pool myself as when i move my goldfish into their pond this summer i don't want them to evict all the frogs from my pond which are helping eat all the slugs and keeping control of mosquito populations etc, frogs will not go willingly into ponds where there are goldfish or koi in them)
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When choosing the place where you are going to put the pond, you don't really want to put it in a really sunny location as then it will most likely get bad algae problems, you also don't want to place it near tree's (particularly tree's whose roots are very invasive or grow primarily outwards from the tree and not downwards). You also want to ideally have it near a source/supply of electricity so you can put you filtration in with ease.
If your garden floods at all or you get a great deal of rain where you live at certain points in the year then you will want to install a drainage system next to the pond to drain away any water run off near the pond (you don't want the pond flooding out your flowerbeds just as you don't want water running off with loads of nutrients in it from your lawns causing big algae problems in the pond etc).
If your soil is heavy with chalk, it would be a good idea to put a concrete collar around the top of the pond hole as chalk soil crumbles easily and it will be difficult to dig a good pond shape that retains it shape over the years well if you have a lot of chalk in your soil. Clay soil though is great for digging ponds in since it holds the shape of the pond together well although i can say from experience its a pain in the bum to shift as its very heavy stuff.
If you are digging during hot weather, when you finish digging the hole you need to put in the underlay and liner in ASAP- we left ours a bit too long before putting in the underlay and liner and the hot weather caused the sides to crack and split, making us have to widen the pond in some area's. The pond sides should be at an angle (not straight up) so they fall apart.
You will need to hire skips for putting all the soil in as you dig your hole, we went through about 3 very large skips when we were digging my 3200gallon pond, so be prepared for these kinds of extra expenses. I spent about £3500 making my pond (i did pay a friend who had experience digging ponds to help us dig the pond though so we could get it finished during the summer months), this includes all of the expenses excluding the pond plants.
VERY IMPORTANT NOTE: you should contact the electric, gas, telephone and water companies and any other utility companies etc to make certain that you do not dig through any pipes or wires by accident- bear in mind though any plans they give you are approximate and not precise.
Disclaimer: i am not a professional, this is just advice given out based on my own experiences and research. I will not take any responsibility for any injuries or if anything goes wrong.
Also..any resource links on general koi ponds would be greatly appreciated.
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The area does get a bit sunny during the late morning, but to the best of my memory it is mostly in shade for the hottest parts of the day ( thus why mum set her pool up there ). I had planned on having a bit of a lip in the pond, dig down two feet or so, and then make a 10 inch lip or so. 
