Some of the universities only accept BMid students on the basis of academic results, so before you go off to do a doula course check what the university your applying to accept. I didn't do any doula training before starting my BMid and I absolutely don't think it needs to be looked at as a requirement. Nice to do if you want, but not imperative.
I think that if you're going to be a midwife then you should do the BMid. I'm not a great fan of doing nursing and then mid if you don't have to. I think we should be really clear that midwives are not nurses. They are midwives. I personally hope that some of the Grad cert courses get closed down to encourage midwives to do the BMid, rather than continuing to view midwifery as a sub-section of nursing. Times, they are a changin'.

Good luck.