Hi,
I replaced an old oil burner for a condensing oil boiler a number of years ago. I've a dormer, around1800sq ft, all double rads within the house - 12 radiators in all. I went with this [broken link removed]model. Also got the heating & water zoned at the time, automatic valves etc, a few replacement rads (purchased myself), thermostatic vales - all in, it was around the €3,400 plus a bit.
There is a calculator for the BTU size of the boiler versus the number of rads, square footage etc.. The one I got installed was the "Popular 26-35kW". Every year, I get it serviced... and so far, all is working well.
Other options are for Grant boilers - similar very good feedback as per firebird, when I was looking at the time - assume a similar price for the boilers themselves.
Also, check if you qualify for a grant at the [broken link removed]