According to Brian Lawson's burials David didn't die until 1881.
Familysearch has them in the census that year:
David MURCHIE Head M Male 80 Ayr, Scotland General Laborer
Janet MURCHIE Wife M Female 62 Ayr, Scotland
Source Information:
Dwelling 28 Barrack St
Census Place Onchan, Isle of Man, Isle of Man
Family History Library Film 1342346
Public Records Office Reference RG11
Piece / Folio 5603 / 79
Page Number 3

I can't see any wills listed for them.

Sue