Best retrofit smart home new build

Take KNX-RF, it saves a lot of cables and their installation. But do you want that?

If he only takes KNX-RF, he could just as well take any other radio system. KNX-RF only really makes sense if you use it together with a "real" KNX, for example as a problem solver, because it integrates super easily into the regular KNX system.

Sure, but he did want KNX. ;)

how difficult is it to program all of this yourself?

if I were to do it myself, would the standard functions already work and I would have to do the automations afterwards, or does nothing work at the beginning?

My knowledge:

no knowledge at all in the field of KNX

I would rate my PC skills as okay to good
I often do more complex formulas in Excel (I know it's probably not comparable)

So it is really not directly comparable to Excel, but it is comparable with logical thinking and experience in event-driven programming.

There is a free demo version of ETS. If you still have time, you could simply try out whether you can manage with it. Have the planning done professionally; during the execution (cabling and programming), you can then accomplish a lot on your own depending on your skill.

Unfortunately, I cannot do anything myself during the execution, at least not with the wiring. The only thing I might be able to handle (if the programming is not too complicated) is the programming. If I do the programming myself, will it not work at all upon handover, or do the standard functions, for example, turning the light on by pressing a button, already work? Which devices can you recommend to me that are good and possibly inexpensive (good value for money)?