I would think that just being interested in Authenticity is a good starting point. Most film makers don't bother...

What help there is on Mudcat might depend on what it is you want to be authentic. There are a number of 'catters interested in dating verifiable dates and printed sources for tunes, lyrics and songs (people singing a song in a period piece, many years before it was written, is often a big gaffe in films).
A quick web search tells me that the murder took place in 1885 and there are photographs of Lee himself and of the house, plus copies taken from contemporary newspapers (www.ianwaugh.com/murderresearch .
The time frame for the events ought to be well documented, but it may be that mudcatters who live in or around Devon may be able to add local colour and background to the area.
It sounds like a worthwhile project.
Quack!
GtD.