Philip, I have looked at a large number of claims about Willem's birthplace. Of most interest to me was a thesis paper titled Huntington's Chorea in South Africa by Michal R. Hayden dated 1979. In this paper he indicates that Willem first arrived in the cape circa 1658, being made a free burgher in 1661. The paper also mentions a possible connection to a van de Merwe family originating in Rotterdam in the 17th century whose descendants are also affected by Huntington's Chorea. (available here from University of Cape Town)
With this connection to Rotterdam in mind I went back to the period maps. A 1684 map of Rotterdam shows an area called "Broek", just east what was then Rotterdam 'proper'. In modern terms it is bounded roughly by the modern streets Crooswijkseweg, Boezemsingel, Boezemlaan, and by the Boezem.
I think the birth suggested on the profile of 1648 is extremely unlikely - with the 1658 arrival date above that would mean Willem was taken on as an Arquebusier/Bosskieter at 10 years of age.
I don't have any facts to support the following, but I would guess to command the higher pay of an Arquebusier took 4 or more years of training/experience and that this would not have started before age 16. (For comparison one study found the average age of arquebusiers in France a century later was about age 40). I would therefore suggest a birth of about 1638 (or earlier) - this would translate to a second marriage at age 30 (in 1668) and a last child born at age 60 (in 1698), both of which I feel are reasonable for the era.
In summary, I would be focusing on looking at Rotterdam or Oud-Beijerland records from closer to 1638.