You could check this site, http://archive.org/stream/cu31924028852998/cu31924028852998_djvu.txt It's a biographical annal of Lancaster Co., PA of early settlers.
It has Rev. Abraham Horst, p. 159; David S Horst, p 517: John C. Horst, p. 279 and Mrs. Louisa Horst, p. 279.
PS Do any of the tombstones have full names and birth and death years?
There is a Rapho Township history at: http://www.raphotownship.com/rapho/cwp/view.asp?a=3&q=430426&tx=1
You might want to contact the Lancaster County Historical Society at: http://www.lancasterhistory.org/ Their address and phone etc. are at the site.
Nearby Rapho, Manheim might also be an information resource at: http://www.manheimpa.com/