Well, just to follow up on my comment, I think I would do the improvements you suggest first (draft proofing, new windows etc, all very sensible) and then maybe see what effect that has on the heating in your kitchen before making the huge layout for a range.
Do investigate better radiator options too. I hear your pain, my current kitchen is freeeeezzzzing in winter, but I still wouldn't get an aga or rayburn put in.
I love my fan oven, and the electric induction hob is a complete revelation, it will be tough to go back to gas actually which was my previous favourite.
The heat in summer is definitely a consideration, our place is very hot in summer and I wouldn't add to that
As far as I remember, my mothers rayburn has a more than adequate sized oven.
I suppose it depends what you will mainly be cooking though. For instance, it's big enough for the Christmas turkey, but generally I think she has to cook the veg first, then put them in the warming oven to fit the turkey in (we're a large family) to cook.
Also, if you will be cooking cakes and things, the temperature really varies a lot between the top and the bottom, so you need to keep an eye out.
The temperature control is ok I think, but my mother's also does the hot water and central heating, so it's usually hot anyway, I can't therefore comment on the length of time to heat etc. Of course, like an aga, if you have stuff in the oven, it's difficult to cook on the hot plates for any length of time wihtout affecting the heat.
If you've never used one before beware! It does take a while to get used to.
In summary, I have cooked on both agas and rayburns, they are pretty similar, but there was a reason our grandmothers switched to gas/electric and if it was up to me, I wouldn't install either.
Sorry, maybe that's not very helpful if you really have set your heart on one...