Is a satellite system still necessary in new buildings, or is IPTV sufficient?

For MagentaTV, you need a separate receiver from Telekom for each TV, which costs a rental fee, right?

Yes.
But you can also buy the receivers externally.

Yes. But you can also buy the receivers externally.

Wouldn't I be better off with SAT-IP if all TVs already have an internal receiver? How is it with SKY actually? At the moment I watch it via satellite. Would that also work with SAT-IP using the SKY receiver?

Yes. However, you can also buy the receivers externally.

You don't necessarily need a separate Telekom receiver for each TV, in principle one device that supports multicast streaming (e.g. Raspberry Pi with Kodi) is enough, then you can also access the channels this way (unencrypted). Many smart TVs basically also support IPTV but rarely multicast, so it would have to be converted to unicast (e.g. with udpxy).

Wouldn't I be better off with SAT-IP if all TVs already have an internal receiver?

Do your TVs really all have an internal Sat>IP receiver/tuner? So far, very few have that, right? A normal Sat tuner is not sufficient.

How is it with SKY actually? At the moment I watch that via Sat. Would that also work with SAT-IP using the SKY receiver?

The Sky receiver does not support Sat>IP.

Do all your televisions really have an internal Sat>IP receiver/tuner? So far, very few have that, right? A normal Sat tuner is not sufficient.

All current Panasonic models have that, for example.

not "e.g." but rather almost exclusively.
Who else has integrated it? The models, outside Panasonic, can be counted on one hand.

SatIp has, in my opinion, never really grown out of its infancy. Especially the servers are notorious for poor reviews, too few parallel streams, or only short availability on the market. That means there is not much choice of dedicated devices.
To my knowledge, CI(+) also does not work. Ergo no HD+ or Sky.

I would simply go for Zattoo or Waipu for €10 a month. Sky via their app solution. MagentaTV is now also available without a receiver and without a Telekom contract. Vodafone has also opened its IPTV outside its own network. The competition is increasing noticeably.