Neil,
I don't think anyone would argue that we should assume that Philip was ordained an evangelist. We would argue that it may be deduced that he was ordained an evangelist.
When we consider that Philip was administering the Word (8:4) and sacraments (8:38), it is reasonable to deduce that he was ordained to the ministry of Word and sacrament, especially seeing that he is later called an evangelist. The only other person called an evangelist is Timothy (2 Tim 4:5), who was a special kind of minister of the gospel, with prerogatives that did not belong to ordinary pastors, but who was not an apostle. In Eph. 4:11, evangelists are listed among the teaching offices of the church: "And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers."
I Tim 5:17: "Let the elders that rule well be counted worthy of double honour, especially they who labour in the word and doctrine." If not all elders labour in word and doctrine, then not all elders are ministers of the word.
As to who the "ministers of the word" are in the WCF, XV:i states, "Repentance unto life is an evangelical grace, the doctrine whereof is to be preached by every minister of the Gospel, as well as that of faith in Christ." So, all ministers of the word are preachers. This is the same group that Paul described as those who "labour in word and doctrine" above. They are "teaching elders."
