First, basic historicity should be largely assumed but wooden literalism should be eschewed. Extracting historical matter from narrative requires practical interpretation, analysis for reconstruction's sake. In other words, be neither defensive nor critical for theo/ideological reasons. If we're going to be trusting or skeptical, both should only apply in the interest of furthering historiographical objectives. We aren't trying to shore up our camp, here. We're trying to analyze the Gospel's content with greater historical sensibility.
Second, notions of divine and human causality must be held in tension with one another, through grace and theological humility. Each interpreter has their own philosophy of History, and their own philosophy of God's involvement with History, but we must accept that Jesus at various moments acted, reacted and was acted-upon, and also that divine power ('from above') was not necessarily absent from nor dominant over any of these particular events, so far as we can tell. In other words, as events actually unfolded, the Father and Son were precisely, and only, two of our many players. And though always the strongest of players, The Divine One does not always appear to provide the most impact on any given occasion. Reconciling these concepts however we may, History's stage must pay proper respect to all dynamic personae, and to all potentially causative factors.
Third, we must draw careful distinctions about what our finished project will or won't claim to be, and thus sidestep traditional fears of constructing a "fifth Gospel" as a new, summarized narrative. What we produce needs to be far different than any Tatian-esque textual rearrangement. A remastered medley of four separate tunes is NOT what we're trying to accomplish. Instead, we'll compose in our own words a song that is technically new, but which succeeds at three tasks: 1) to faithfully capture the spirit and soul of our source texts, 2) to represent both Gospel content and contextual 'background material' holistically, and 3) to provide greater awareness and insight into aspects of texts that we should already know, but often fail to recognize.
Fourth, the purpose of writing this Gospel based History is not to discover something the Gospels didn't already tell us, but to build upon and make more from what they actually do tell us. In other words, reconstructing sound History is a lot like constructing sound Theology, except that Historians naturally ask different kinds of questions. We will primarily focus on scripture's testimonies about practical happenings and examining how persons who carried and passed along their ideas and beliefs, how they took action and interacted with one another, and how they did so in ways that may or may not have been fully in line with anything God Himself might have been attempting to do, at particular times.
All in all, a Historian of Jesus' life needs to believe in the texts of the Four Gospels, but analyze those texts historically. She must read, consider and comment on them while asking different sorts of questions than theologians typically ask. She must write different sorts of overviews than theologians have usually written. Like any good Theologian, she must build up and make more of scripture's God-breathed content, in ways that neither add to nor take away from scripture's claims, but which enhance what is already contained there. The Historian must engage with historical issues without ignoring theological truths, and construct narrative summaries without ignoring the deep perspectival distinctions of Matthew, Mark, Luke and John.
Like any good work of Theology, a Gospel based History should impact readers by making them *more* eager to dive into the scriptures, not less. Such a project should neither be calculated to inspire a radical new vision OR a refreshed reinforcing of traditional views on church history. Instead, a Gospel History project should be expected to render fresh four-dimensional (ie, fully spatial & temporal) perspective on historical facets of the original Gospel Story - especially on the most living and active aspects of that holy scripture. Properly situated, the goal of any such work will be merely to bring out more fully the actual vibrancy of the One Story which is already there to be found in the four irreplaceable Gospels.