I have a bow thruster in my 320. Don't know the specs, (HP etc) all I can tell you is that I have 4th battery to run it (same size as the other 3). It is nice to have and can help in a light breeze, but it gets overpowered in a strong breeze. When I bought the boat, it was my first in-board and it was a comfort to have. on my second day of owernership I tried to use it to pull my bow into a fuel dock during a heavy breeze and it didnt have enough thrust. I got blown away then laughed at.. (thats another story)
After getting comfortable with inboards, now, I don't need it.
It is nice to move the nose of the boat around at the dock, when getting lines, but not necessary. Now I only use it to exercise it.
Bottom line, nice to have option on a new or used boat, but I wouldn't recommend investing the money to retrofit one.