I haven't one myself, only an LPG condenser. Oil condensers are bit more expensive then gas condensers due to the technical layout and the material choice. Because of the sulfur in the oil sulfuric acid will condense together with the water, so the metal parts have to be acid proof.
Check the Sedbuk page for a choice of manufacturers, and make sure the type you buy is a "modulating" one. http://www.sedbuk.com/
From a personal view I would opt for a cheap one with a long warranty.Since you have a solar panel installed the running times would be shorter compared to the standard home heating system and therefore the life time of the boiler will be extended.€ 4000 would be bit hefty. Usually there is 20% off (reduction) when you ask for it. Shop around.