I've seen one in all of 2017 (out of ~20 companies).
> Is it an internal or external person?
In that case it was internal
> How technical are they?
More legal than technical, but that's a very small sample.
> I'm a developer and I have a law degree; would that put me in an advantageous position to become one?
Yes. In fact that's probably one of the most lucrative combinations of fields.
> Is there a market for 'consulting DPO's', like companies hire accountants, if that's allowed?
YES! In fact if you are halfway decent this would be an extremely lucrative thing to do, but it probably will become less so over time as the knowledge gets diffused.
1) It was not their main role
2) It was a token gesture, someone to address post too!
Every consulting firm I know is trying to sell GDPR services atm...