Open fire is in my opinion always bad idea. apart from all the dirt and smoke (unavoidable), you also have suboptimal heat distribution.
There's brilliant stoves out there that take air also from the outside (so keep the property airtight), and use fans etc. to distribute the heat into the room properly.
I am personally not a fan of air-sourced heat pumps, but it might be appropriate for this climate. My preference would be ground source - either a nice 100m straight into the ground hole, or if you have enough land, the appropriate area for the piping. Main reason for me is that ground temperature in Ireland (a few meters below down that is) is constant around 11-13 degrees.
Can't help with the cost effectiveness, but there's a ton of information available from various manufacturers.