House with ground floor and upper floor, do you know any recommended house planners?

Take Sat. Guess how often the DSL has gone out here. Felt like every 14 days for a few hours. SAT almost always works, only not during thunderstorms or snowstorms.

Are there online house planners that ask all the important questions?

The ones that plan a house when you provide size and number of inhabitants, etc.? – no. Configurators that filter the huge selection of all models are available on every prefabricated house manufacturer’s website. That helps with orientation, even if you want to stick to stone houses.

The best way to go to an architect is with a room list and possibly rough sketches – but it won’t get cheaper if you try to pre-draw the house for them ready to stamp with Maggi Fix 3D for Windows ;-)

Land plots cost around 100,000 EUR here.

What kind of plots?

The plot is the host who can throw a wrench in your plans: favorite roof shape not allowed, or similar.

150 sqm as a "one-and-a-half storey" needs about 125 sqm of footprint; with a site occupancy index of 0.3 and floor area ratio of 0.4, for example, that means a 417 sqm plot.

Inexperienced people easily underestimate what extra effort a hillside location entails – often it is economically not suitable for "without a basement" at all. Unfortunately, this relativizes many seemingly cheap plot offers.

It should also include network cabling,

Structured cabling, done properly, yes. But don’t let yourself be sold high-pressure gold-plated Cat.Elevenpointsomethingplus cables; you can overdo it. Ducts are also important (not in the sense of tunnels for cables already planned, but truly empty for cables you can’t even think of today).

And: today is yesterday’s tomorrow – copper is current, but the future isn’t far off anymore. So keep fiber optics in the back of your mind.

Correction to above. Window cuts 110, not 113. Remembered incorrectly.

No, correct: nominal width "110" as outer frame dimension means rough construction dimension 113.5.

Which 6 rooms would you like? With that number, it will be tight with 120 sqm depending on the size of the rooms ;-)

Most of the time it is the financing that dictates everything else. At least the choice of an affordable home builder and the technology. Talk to everyone, absorb any information. Check out the pinned posts here about construction costs.
Listen to what the sellers tell you, but don't believe everything. Many are after a quick contract. Please don't sign anything there. But listen, read... and in three months you will have a picture from the puzzle pieces.

We are still here too :)
But first, read a lot.