To be wise is (often) to listen to advice of those who have gone before you. In the last week or so there have been several posts giving guidance to those interested in PhD studies that I thought I would link here (especially since I am one of those people so I may want to return to these links myself):
(1) Daniel Kirk argues for the M.DIV as the best pre-Ph.D degree for seminarians here.
(3) Kevin Chen, who has recently completed his Ph.D, describes several things he would have done different/things of which current seminarians need to be aware before doing a Ph.D here.
(4) Ben Johnson, who has just begun working on his Ph.D at the University of Durham, gives some advice for those transitioning from a Th.M to a UK Ph.D here. He reminds us of some older, but still very useful blog posts by Ben Blackwell and Nijay Gupta as well.