Piece of string question I'm afraid.
But, maybe some informed conclusions can be made ... however, you need to gather some information.
Concentrating on you first question about oil consumption, some questions for you:
1. Does the current way you run the solid fuel range result in a comfortable house etc? Are there any issues with this set up?
2. What is the efficiency of your current range and the new condensing one?
3. How much fuel do you currently use (and what is the fuel mix) during the heating season and per year?