Should I hire a structural engineer or an architect?
At least in the USA both architects and structural engineers will do this type of work.  Not all architects will want this small of a job (but you may be surprised how many will) and not all structural engineers will want to draw up plans.  Basically you are on the right track; I would just call around to the architects and structural engineers in your area and talk to them about the job.

I have used a structural engineer in the past but I did not have him draw up plans (it would have cost more).  He come out and looked at the situation and wrote a report on what was needed.  However my job was even smaller than what you are discussing.  I would start with local architecture firms and see if they will do what you need, if they don't want the work they may refer you to a local structural engineer.

The important thing is that you get plans created by (or at least signed off by) a certified architect/engineer.  This should make getting a permit really easy and you will know you are doing the remodel in the right way.