It's cheaper to build second storey on top of existing bearing in mind it is usually constructed from timber frame and cladded then rendered/finished to your taste etc. What I have been told the cost blowouts tend to occur with the level of finish you want internally ie -bathrooms/wet areas can be finished modestly or they can get hugely expensive - tiling and plumbing.
We were quoted $120-150k a few years back to do this, similar to your idea of a couple of large rooms and a bathroom/wc.

You will also lose a large chunk of space somewhere in your bottom story to allow for the stairwell and landings.

We were given no indication of foundations being an issue it was the roof structure we were told is the focus of the structural engineering.

You also need to factor in the exterior finish of the top story needs to match the bottom so you may need to render/paint bottom story...

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Bye bye betty blue. Hello F6.