Congratulations LED.
I'll add my own experience for you to think about, even though it's long out of date.
We had a large, three storey new build house and we chose to put in a mains gas fired, 2 oven Aga which also did the hot water. Having it heat the water meant that we could claim the VAT back in with all the other new build claim, but we wouldn't have been able to do that if it was cooking only.
We did also have a combi microwave. With hindsight we'd have been better forgetting the microwave bit and gone for a separate small cooker so that we had a separate hob for use during the hot summer months, or when we came back from longer absences. We had an immersion heater fitted to the hot water tank, so we had options.
I'd never used an Aga or other range before, but DH particularly wanted one and I didn't mind. We both loved it, although not so much as we moved in Christmas week and it blew out on Christmas morning - one side effect of cooking everything in the ovens is that you rarely get any cooking smells (it's vented directly outside) so it was as well we were checking. Also a good kitchen timer is absolutely essential incase you forget what you've put in there and by the time you remember it's charcoal.
We bought original Aga pans and kettle and le Crueset heavy cast iron casseroles and they're all still in daily use 18 years later. The moral of that being the stuff's expensive but you certainly get your money's worth.
The 'ironing' is fantastic, more 'pressing' really, but just beware of your folded items draping over the lids, or things over the rail getting too close.
DD's dark blue jumper once developed some unexpected orange stripes 
Make sure your vendor leaves the cooling plate (used to lower oven temperature) and any pans or the toast grid that she's happy to let you have. And yes, do get some really good quality oven gauntlets to protect your arms.
When we had ours the doors simply lifted up from the 'hinges' for cleaning and the inside of the oven you just sweep out with a wire brush.
We really enjoyed the food we cooked in the Aga, there are untold amounts of resources in books and online, which will help you get the hang of things, just set in your mind what the technique is and you'll be fine. The food does seem to stay more moist as well, which is a bonus and it's great of course for cooking in advance to freeze things.
I think you get a lot more creative in your use of what is more than an oven, because it's on anyway, you find all kinds of things to utilize its warmth and it will help with the large kitchen.
We don't have an Aga where we are now, but have gone for a different manufacturer, Everhot, which is a programmable range where you can set temperatures and it turns lower overnight. It also has a grill element, which our old Aga didn't have. Our electricity monthly payment is £133 for a very large house. The oil bill doesn't even bear thinking about, but that's heating only - not the range. We've been lucky most of this winter that warmth from the range has meant we haven't needed the central heating on quite as much, so it is a bit swings and roundabouts for us with the bills.
Hope your purchase goes smoothly and that the above might be of some help.
PS if you do much decorating, the Aga lids with a cloth on are fab for drying out your cleaned brushes.