Looks like a nice space for a pond, I wish I had that much room and privacy. Looks like you have good access to the site and plenty of elbow room so anything is possible. For starters the dimensions you list will result in a pond of about 5,000 gallons which is quite an undertaking. Probably need a backhoe for digging out the pond and various plumbing/electric trenches. Plus somewhere to dispose of soil. Might be a good idea to think about future landscaping projects (planting beds, retaining walls, etc) before hauling all that dirt away.
Here are some general things to think about, even though you mentioned some of your preferences already.
Will this be a koi only pond or do you want plants also? Koi can be pretty rough on plants, so you may want avoid one or the other. Goldfish can be a nice alternative if you want alot of plants.
Rubber liner or natural/clay bottom? For this size pond you are probably best going with a 45 mil EPDM liner. If this is going to be a liner pond, then you may want to adjust the sizes according to the most common sizes of liner available (15', 20', 25') rather than having to buy a larger liner just for an added foot of width.
Rock bottom or bare liner? A rock bottom pond does look nice and much more natural than a bare liner, but rocks tend to trap alot of debris and can be a maintenance headache. There is alot of debate on this issue so I will leave it at that.
Straight walls or sloped? All walls will tend to settle over time depending on soil types and construction methods, but straight walls usually provide more protection from the predators that you will have like raccoons and herons. However, straight walls usually need to be reinforced. Common ways of doing this are either block wall construction or at least installing a concrete collar around the pond to help stabilize the edges. The other advantage of straight walls is that they maximize water volume for a given circumference.
Filtration? Filtration needs are based on water volume and fish load. If you are happy with just having a few fish, then you can get away with something rather simple like a skimmer and bio-falls, but if you want alot of big koi, then you may need to plan for something more substantial. There are alot of good DIY filter designs available. Also remember that you will need to get power out to the pond for pumps/lights/etc.
Dealing with run-off water? With that location you will need to keep run-off out of the pond. This can be accomplished with proper berming or drainage. Another option is a raised edge pond. This is a more formal look, but saves on digging and will keep the floods out of the pond. Also helps in predator control.
Not trying to scare you, but there is alot to think about. There are a number of good web sites out there to get information from, the trick is finding the one that fits your idea of what this pond should be. Everything from very high tech koi pond, to much more natural looking.