Regarding preliminary sampling: When building with a larger company that has a sampling center or a sampling day or something similar, one should always insist on preliminary sampling before signing the contract. This is usually not done gladly, as the company has to invest a lot of time in it.
If the company, on the other hand, works more with local craftsmen (as was the case for us), it is, as described by , advisable to get a list of the craftsmen and speak directly with them.
I would also like to put forward an unsubstantiated claim: The cheaper the provider in the standard offer (house price), the more expensive the positions during sampling become. (This results simply from the larger difference between the presumably cheaper product from the standard and the desired product of the target equipment compared to a standard product from a more expensive house offer).