you can keep the existing rads. Do you have gas piped to your estate?
If so, I would imagine it would be about €3k at least. If you give Bord Gais a call they will send out someone to give you a free estimate. I got one a few yrs ago and it was €2.5k. I even have the pipe going from the junction box outside my house to the area in my utility where the gas boiler would go. Even with this work not having to be done, it was still 2.5k