Sorry I can't really help out at all but just wanted to say that just this morning I was looking at a brochure for a company that updates kitchens as I am thinking our kitchen could definitely do with a lift too. They replace granite bench tops and replace cupboard doors/drawers leaving the existing fitted cupboards etc. (basically, exactly what you are suggesting). Not sure where you are, but they are in Mornington (called Granite Transformations. Sorry, not sure if you are allowed to post business names, so if that gets deleted, PM me). ETA - just realised they are in Melbourne and quite a few other locations across Vic too (and Australia for that matter, I think).
Alternatively, for the kitchen and bathroom, you are probably best just to start getting quotes to get an idea of prices and what is 'doable'.
Also, I think (??) that you would select the gas top first so that when they are making/fitting the bench they have the right measurements to cut the space to fit the appropriate appliance. But I am NO expert, so please don't go on my advice alone.
All the best.