Best retrofit smart home new build

does it not work at all when passing it on or do the standard functions e.g. light on by pressing a button already work?

It depends on what you agree on. If no parameterization is agreed upon, then nothing will be done. However, the devices can basically also be fully parameterized before installation.

Which devices can you recommend to me that are good and possibly inexpensive (good price/performance ratio)

It depends on what functions are desired. It is generally the case that every manufacturer includes additional features and those are the differences. You may then urgently need this or that function at a certain place. But as a guideline, MDT is very popular in Germany... in my opinion, however, I would only take their built-in devices for the cabinet from them. The glass push buttons are functional and inexpensive but rather an eyesore and in terms of haptics just like glass usually is.

The choice of devices always depends on the device itself. I mostly prefer MDT for actuators because the applications are pretty great, but for example, when it comes to high-voltage dimmers or IP gateways/routers, I use something else. Not to mention switches and sensors.

If you just have everything installed and then do the configuration yourself from scratch, nothing will work at handover. If you use actuators without manual operation, the installing electrician will almost certainly refuse any acceptance or functional testing because, for example, they cannot test shutters. You simply have to talk to the people and consider such things.

There are plenty of "KNX courses" or videos online that at least deal with the basics. The KNX Association also offers basic courses that might even give you discounts on ETS if you complete them. Matthias Kleine’s videos are also great, and Voltus has KNX tutorials for complete beginners. Just get involved with it. Only very few will manage to buy the software and start right away and reach their goal fairly quickly. The money you save, you have to invest in plenty of your own time. That’s the deal. Nothing is free. But that applies to almost every system; intensive engagement is required also with Loxone, Homematic, or others.

The glass switches are functional and inexpensive, but rather an eyesore and have the typical feel of glass.

I also mostly prefer MDT for actuators because the applications are pretty great, but for example with high-voltage dimmers or IP gateways/routers I use something else. Not to mention switches and sensors.

I currently have a box of MDT actuators for the distribution board here and ordered a glass switch as a trial. To be honest, I find it quite stylish and it is also frequently praised in the KUF. I cannot make comparisons to switches costing 500 euros each and in my opinion, one should not make such comparisons.
For presence detectors and other sensors, however, I will switch to other manufacturers.

Button 55 or the smart version with display and tactile click-clack I find top notch. For €60 a double button with multiple functions, group control and status LEDs in RGBW. What more could you want, they also fit perfectly with the A550 frame program from Jung. I am quite a fan of MDT, they have really improved.

Yes, a matter of taste. I simply don't like the well-known glass switch, neither visually nor tactilely. There are far prettier//better ones, of course. But clearly, they are not in the same price range. You always have to look at the applications as well, what is desired/needed.

No question, MDT is right at the top here in this country and they did everything right.

And besides, they are just more likeable than the established electronics giants (says the rebel in me). :)