I have done some research on the Internet and found the following:
David Evans married Mary Kepheart on 17 October 1850 in Belmont Co., OH.I could find no marriages listed for Jefferson Co., OH.The birth date for Mary of Sept. 28, 1834 in Philadelphia is the same date you have.If this is our Mary Capehart, then she and her husband migrated to Iowa before her father, Andrew, & siblings did.They are suppose to have migrated to Iowa in about 1844.
I did some census research and in census dated 5 August 1870, Deep River Twp., Poweshiek Co., IA I found listed a David Evans age 48, Mary age 36, Atthena age 17(?) spelling (maybe this was another name for Rosella), Clarence age 15, Lydia age 9 and Sarah age 3.The year you have for William's birth would make his age in 1880 census about right.
I did some census research and found a David Evans, wife Mary, daughter, Sarah and son William living in Deep Reiver TWP, Poweshiek Co., IA on the June 12, 1880 census.David was listed as 58, Mary 46, Sarah 13 & William age 6.It further states that David was born in OH and Mary in PA.David's parents were born in OH and Mary's in PA.
I have never researched William Evans and his wife but will do so.If I learn anything I will let you know.