I would gather together as many secondary sources as I could to see how Wilberforce has been regarded by past historians. At the same time, I would take careful note of the primary sources those historians used - and I would see if you have access to them. I would then carefully read those primary sources with an eye to source criticism (i.e. what points of views in those sources affected how reality may have been twisted to serve the author).

What that body of information, you should have the basic nuts and bolts to begin writing your version of the story. That having been said, you should carefully consider historians who have discussed Wilberforce and ask yourselves how your treatment and point of view will be different. I only write books that I need to read; I don't write books that I want to write when others have done the job already. Coincidentally, publishers will consider the former, but they are not likely to consider the latter without a unique point of view, conclusion, insight ... or something.

You might also consider the career of William Chrystal, a minister (ret) - historian (very active) who has written extensively on the history of religious figures and their points of view. He also a Hamilton expert - but that is another facet of this articulate scholar. Sometimes, it has been an uphill battle because history and those who publish history do not always consider devotional subjects. I hope that helps. Good luck - nice topic.

When dealing with subjects outside my area of expertise, I often look at the Oxford Bibliographies, which are generally up-to-date and thorough. They'll point you toward some of the most important books about a subject, and give you a bit of info about them so you can decide if they're truly relevant. It's generally better for bigger subjects (e.g. abolition) than smaller ones (e.g. bullfighting), but it's worth a check.

You might need institutional access for some of the bibliographies. It should work if you log in when visiting a local university library.