how2girl is right, ducted is better. Ducted extracts better and with less maintenance. But it might be difficult or impossible to do, for example in a two storey house with the kitchen in the middle so that there is nowhere for the ducting to go. suzineedsahouse is right, different people have different requirements and what is important for one person might not be important for another person. I have seen a house where a window beside the cooktop was enough for the owner (I did wonder how that works on windy or rainy days). Some people are happy enough with a non-ducted rangehood. Some people insist on a ducted rangehood and are willing to rearrange their houseplan if necessary to ensure they have space to install one. I have heard of people who have chosen commercial rangehoods for their home. I know of people who have a second kitchen outside in their alfresco area which they prefer to use for some of their cooking and sometimes even those have a ducted rangehood. (Btw as far as I know a gas cooktop only affects your choice in regards to how high above the cooktop the rangehood must be.)

I would highly recommend AVOIDING a siemens oven. We bought a normal oven, along with a combo steam oven, about 6 years ago. The whole time we kept getting faults on our steam oven and after every fix the error would come back, or a different error code would pop up. After 3 years the top heating element fell loose because the screws holding it in place completely rusted and left a hole into the oven cavity. The oven had to be replaced, which was a nightmare to organise, but we eventually got them to agree/organise. Another 3 years down the track and we have had the same issue with the rusted cavity top! I have since found other reviews stating the same issue. We had issues with our siemens gas cooktop also, however in fairness, after a service technician fixed those it has performed well enough. You would expect, paying so much, that an oven would last more than 3 years. We designed our new kitchen around these appliances, and our other oven will now not match a new steam oven. They have offered us a refund of our originally paid price, which is far less than what the oven costs now. I'm extremely disappointed, we will be left substantially out of pocket and with an oven that does not suit our kitchen. Aside from that, their ovens don't even cook well - things don't seem to crisp up as well as they used to in my much cheaper previous oven. Don't be fooled by the sleek design like we were.

This is an Australian site, we work in centimetres. So 30" is 66cm,and 36" is 90cm. Assuming the oven is on the 18ft side, then I would definitely go 36". Do you get a 24" oven or 60cm oven where you are? You could install two 24" ovens. Then you only need to heat up a small oven for everyday use, and you have the option to run both ovens when cooking for a crowd. All the best.

if you mean a 60cm wide built in cooktop with separate oven then there would need to be a new cabinet to enclose them so it makes sense to have a new cabinet to fill the whole space now used by the stove and the existing narrow cabinet but don't understand the need for a deeper edge and custom made real stainless steel counter would be much more expensive ...consider a chopping board timber counter and/or contrasting laminate (or faux stainless steel?) for the new counter and surrounds for the oven and door... the challenge will be the round edge on the old counter so could also be interesting to ask the cabinet maker about swapping the rolled edge for a square edge in the new laminate colour or vintage timber edges?