1. A good pond kit is one that professional installers will use. In the past 10 years there has been a really large increase in the installation of koi ponds for homeowners. I think this is mostly due to the increase in the home improvement shows on cable TV and all of the great backyards they show. Since the hobby has increased so fast you need to be careful as there are a lot of cheap knock off products out there that are made out of cheap crap. Take for instance skimmers and waterfall boxes. 90% of them are made out of cheap plastic and the sides can bend and bow without hardly any pressure. Some of them are so cheaply made that the tops won't even fit well when they are brand new. Plus the pumps and the PVC plumbing in a lot of the kits are crap. Pond liners are either rubber or plastic. With a rubber liner you can't go wrong, but with a plastic one you need to be careful or you can wind up with a good one or a terrible one. So, unless you know the difference stay with an EPDM rubber one.
2. Jets if properly installed will increase the aeration like a whirlpool bath. The only problem is that you need to punch a hole in the liner to install them. This is just asking for trouble down the road should one leak from expansion and contraction in summer/winter. A good aerator will accomplish the same thing and every pond should have one.
3. Do we need a bottom drain? No you do not "need" one. Again, punching a hole in a perfectly good liner is just asking for trouble. If you are really experienced at working with liners and bottom drains then that is another story. But as a first time ponder stay away from bottom drains.
4. Some web sites advise against building "shelves" in a pond due to several reasons, predators, not easy to clean, etc. Should we use shelves or just build a slope down to our maximum depth? If you are going to have plants you need planting shelves. Plants are part of the backbone of a good ecosystem to remove waste from your water.
5. Can we keep plants in a koi pond? Many sources say koi eat the plants and thus it's difficult to keep them together. If so, how do we provide shade for the koi? Plants, plants and more plants. Yes they will nibble on plants they like such as water hyacinth. But you can always keep replacement plants growing in the top of your waterfall box or penned off in areas the koi can't get to.
6. What is the difference between keeping koi and goldfish? Also, what is the difference between a $1000 koi and the $5.99 koi at PetSmart or Petco? There is no biological difference given that both koi are healthy. However the "mutts" of the koi world are a dime a dozen and the ones with really good markings are rare. See this section on koi markings and judging: http://www.pondkoi.com/koi_markings.htm http://www.pondkoi.com/judging.htm
At some point you will not look at them and see just pretty fish. You will see Kohaku, Showa and Sanke. Once you learn to appreciate the different markings then you will magically develop a need to start keeping specific types. Some people are perfectly happy with "mutts" and never get hooked on the more refined koi. Some people really get into the hobby and would never think of having a koi that didn't have any good markings.
7. Should we use rocks on the bottom of the pond to cover the liner? Also, where do we look to buy rocks and what should we look for, especially for the waterfall? Waterfalls should have stacks of rocks with pieces of flat slate or something else flat to provide staged drops. The longer the stream the better. You should use a mixture of medium sized gravel on the bottom to hold the plant roots and to protect the liner. You can get rocks, sand and gravel delivered to your driveway from any aggregates supplier in the yellow pages. It would be a good idea to visit them and hand pick rocks if they let you. Also that is the time to pick out your pieces of slate instead of having it randomly delivered.
8. Lastly, is it really difficult to build a pond (plumbing, electrical, etc) or can we do it ourselves? We are fairly handy and, as I said before, would like this to be not only a project but a new hobby. We have several aquariums in our house and would like to beautify our yard with a pond. The hardest part is the digging and hauling large rocks around. The rest of the work is easy. The plumbing of a system is very simple. Just put the pump in the bottom of the skimmer and use high grade flexible PVC and route the line to the waterfall. Very easy.
All forum members can buy the kits we offer at the same price that we sell them to pond builders for. But you have to promise to take pictures and update your build on this forum as it gets done. http://www.pondkoi.com/pond_products_pond_kits.htm
I can guarantee you that these kits are the best you can buy and there is nothing in them that is cheaply made.