If I were to place my microwave over my cooktop, this would be how it would look. Personally, I don't feel there's enough room between the bottom of the microwave and my cooktop to be good. My cabinets over the cooktop are smaller so it can happen, but it's not a good look.
Also, since the house has cabinets there now (as my house does), I'm betting that your cook top ventilation system is not an upper hood based one, but a "downdraft." That's what I have also. The exhaust for this cooktop is underneath it in the cabinets (that I also use for storage of my pans). I can get a range to go in it's place but I get about 3 choices of downdraft ranges.
Worse, is that the kitchen you're looking at has NO exhaust for the cooktop. Sometimes you need it.
Also, where I work, we have large microwaves that are also dual convections. When I lived overseas and had space issues in the apartment and no real oven, we bought a large microwave/convection. I am not a huge cook but I do use my ovens. For instance, I make easy sheet pan dinners, roast vegetables, etc. Nothing major, but I wouldn't be doing it in any convection that I've ever had.
One of my two wall ovens is a convection oven (basically it has an internal fan that circulates heat). It's supposed to be better for baking. I haven't really noticed a lot of difference but there is some.
I still don't believe for a minute that the convection/microwave combo will do the same work as an oven having had both.