Suggestion: Send these photos to Mike Otto along with a request to chat briefly with him. He's co-owner of Pond Boss, an expert in his field of pond building, and an all around incredible guy - he will review and give you his recommendations on how to start. If your heart is set on this property, I feel you should do the following in regards to the pond:
1. Email/call Mike Otto request brief meeting
2. Follow his instructions, which will probably include contacting an engineer and pond construction professional for a bid
3. Submit bid[s] to seller and ask for consideration to share costs
All the other info here is important too...but has been covered by everyone already. I'm focusing on the elements I am familiar with as I've been through this same process.