But wait: I haven't quite answered the question yet...!
The actual question you have is about a standard way to communicate estimations. You even include a format you'd currently go for.
First up, a lot of my text above can be summarized as these directs points of interest in your current setup:
- A high level / one-page project description is still great, though perhaps not for estimations and feature lists;
- Don't talk about "Redesign", instead focus on a "New Product X" (that coincidentally does a lot the original product does as well);
- Setting a Scope seems like a very smart thing to do (though you may want to call it "initial scope", as the business people can and should change their mind along the way, be it at a cost);
- "Description of tasks" I'd replace with a high level overview of features, but whatever you do leave out the hours estimations.
- Don't include a total estimation here, unless you directly refer to the other docs (see below);
- The "Risks" section is great. You can add the suggestion of doing that work asap, so you can asap change directions or cancel the project if needed.
- Don't go on the defensive like you do in the last part, but instead (a) be clear in your docs that you estimate and not promise, and communicate outside informally what you feel is the difference between the two.
Now, to finally answer the question, the standard way to communicate estimations I'd recommend:
- Utilize Story Points (or similar construct) and Velocity to convey estimates;
- Create a Product Backlog for a high level prioritization of features;
- Create a Sprint Backlog for each iteration with the next few product backlog items to work on;
- Create a Burndown Chart to show progress and estimated time left;
Most Agile methods (and Scrum in particular) have the above artifacts in one way or another, and I highly recommend using those for your purpose.
Again, I recommend reading Agile Estimating and Planning because it basically is a long answer to your question. It introduces all of the above artifacts quite clearly. Searching online for those tools will also help.
Good luck!