First, I recommend not making any exterior changes for a year or more. You will find many benefits of the way it exists, besides getting the inside in shape for your desires.

2nd, a slight slope is good for drainage and landscaping plants, and foundations. Koi ponds are expensive, and highly desired assets. Maybe fence it and allow it to run naturally until you learn a lot about it. If you think of it as an ecosystem, and allow it to nourish plants, the fish will take care of themselves and you would have a nice balance. Consider joining an online pond group.

Mike www.SolarPanel-Rental.com

I agree completely with last post. I'm a big fan of living with a house for awhile to see how you use it, what you really want. Work on the inside first, look outside and think about the views you want, how you'll use the yard, then tackle it next year.