Are some of the Suttas we have today different from the original Suttas as taught by the Buddha? Are there any surviving pre sectarian Buddhist sources and commentaries? What research has been done on pre sectarian Suttas?
AFAIK, nothing can be definitively sourced back to the Buddha.
With that said you have...
The Ghandaran Buddhist Texts which are the oldest Buddhist writings yet discovered. This doesn't mean they're the words of the Buddha, but if you go by the assumption that older is more likely to reflect the founder's intent (not necessarily true!), then this is one path.
Various historical studies on Early Buddhism, including this page. These studies can get technical and may not be satisfying to a practitioner. They may also spend a fair amount of time studying the context of Buddhism. Another source (I don't know if it's early enough) is this book, freely available online.