What's your question? Do you want to buy a pump or install a new pond? Full sun is great for the plants. In my case, the pond came first (2,500-3,000 gallons) and we purchased the proper sized pump that could recirculate that amount of water. Only you can answer if you love ponding enough to take it to the next level. Being in a very cold zone like you, I would suggest in-ground. Our summer season isn't that long and in-ground would extend it for you. A bigger pond also gives you more opportunity to add fish, do some water gardening, etc. It should also be at least 3 feet deep so the fish can overwinter in the pond.

Basic 101:
1. Think where you will enjoy it, visually and the sounds.
2. Whats going to be reflected? Trees or ugly neighbor shed?
3. Water Garden, Koi Pond, or hybrid?
4. Depending on above answer what depth, size, shelves, etc...
5. Don't get too crazy on shape, can get dead zones.
6a. Size of fall and stream will tell how much pump. 1,200 to 1,500 gph per foot of width on stream and fall minimum.
6b. You want to circulate volume of pond minimum of once per hour, more if koi are involved. This is the minimum, visual effect of stream and fall more important.
7. Most pumps that are specific for ponds are solid, I prefer Danner, Easy Pro, Tsurumi. Tsurumi only for high head!
8. Consider head! 10ft. horizontal = 1ft. vertical, 1=1 for vertical obviously. Consult specific pump.
9. In ground or above is basically personal preference. Above ground is more cold and changes treatment over winter, but is more unique in most regions.

10. Rock or no rock. Personal and a hot topic. Personally if you don't have massive filtration I feel rock bottom ponds become maintenance nightmares, but many do not feel this way.
11. CONSULT A LOCAL PRO!

Hope that helps.

Certainly the pond comes first. Decide on the size, depth, etc. Then you can figure the water volume. You want to buy a pump that will easily turn the entire volume of water over every hour. For my 1000 gallon ponds with a waterfalls, I use 1500gph pumps. That size accounts for the water volume and the lift of the waterfall. Smaller pond? Smaller pump. Pump size is determined by the amount of water to insure a healthy pond. Make sense?

One thing I would suggest is - consider the sizes of liners when you are planning. I designed my pond and dug the hole, but then when I went to get the liner I found they were available in widths of 15' and 20'. Mine was slightly too large for a 15'x 15'piece so I had to get 20'x 20'. When I put it in I realized it was WAY more than I needed. I decided why waste this extra liner and that I should go ahead and dig out the pond larger and add this shallow area around the perimeter. Which meant WAYYY more work because I had to undo a lot of my original cinderblock construction.

I'm still not done with the freakin thing. If I'd considered the size of the liner from the beginning I could've designed the pond differently from the beginning and saved myself a lot of work.

The pump you have some flexibility with, you want to have a little extra power to it anyway.... for example, I was advised to get a 1200 gph pump for my 700 gallon pond. Actually I think I'm going to wait until I fill the pond and measure the exact amt of water before ordering the pump, just to be sure.

your ponds entire water volume should be overturned hourly thru your filter at a rate of at least 1 - 1 1/2 times the volume of water per hour.. a 500 gallon pond should have a pump of at least 500 gallons per hour or even a 750 GPH pump if your filter is located higher than your pond water level.. a good rule of thumb... You can NEVER have too much filtration on a Koi pond. NEVER!! so don't skimp on the price of your filter/pump setup..