I think the reason he initially targeted prostitutes was because he saw them as an easy target as they were easy to lure into his car and take to secluded locations. The first 3 attacks that he was convicted of / admitted to had no links to prostitute. This included 14 year old girl who was walking down a country lane near the farm where she lived with her family. Thankfully they survived as he was interrupted.
I find it hard to believe that he genuinely thought that Marguerite Walls was a prostitute, she was attacked in an
upmarket residential area outside of Leeds, nowhere near the light district. The police didn't link her murder with Sutcliffe initially because he used a different method to kill her . They reported the murder as sexually motivated as like many of sutcliffes other victims she had injuries consistent with sexual assault.
If Sutcliffe commited his crimes because God told him to "clean up the streets" he would just kill them, he wouldn't inflict sexual injuries on his victims or ejaculated over them as they lay dying. When he was arrested he was also found to be wearing custom made undergarment designed with a hole in the front designed in the front to give easy access to his appendage.
Sutcliffe brother claimed in an interview that he had told him that he had lied about his paranoid delusions in an attempt to receive a reduced sentance in a cushier environment. This all casts doubts on Sutcliffes claims that he did this due to having paranoid schizophrenia and wanting to do God's work.