Are you sticking with flush doors? If so and you want to paint it, get the cheap flush fire door (there are also 4 and 6 panel ones that would be around the €80 - €100 that are designed to be painted)
If you want a timber finish the cost will depend entirely on the timber type and door design and could be anywhere between €80 - €600 +
If you are looking for Oak in particular I would go for an engineered Oak core with Oak veneered surface. Don't worry about getting half hour rated if you go for this type of thing as the door will be solid anyway.
As mentioned in the other current thread on here about internal doors, Deanta and Doras would be a good starting point.