Getting a bit more confused…
After further reading I found a mention in the blog that DIY-products will be discontinued. Ok, fair enough, so HW-rev 2.1u seems to be the last version available,
Searching in the forums I found that the relay functions was removed in rev 2.3 and is not available in the DC version. Sad.
Reading the OsPi manual I learned that the recent OsPi-version seems to feature analogue ports for humidity sensors e.g. now, so the website is obviously outdated.
Searching the forums I found some threads discussing the LCD screen for the OsPi, so it seems possible.
However, I guess I will go for the DIY-AC-variant then, seems to fullfill my needs.
I only wonder if it was not only possible to add a 433Mhz transmitter (e.g. to control sth.) but also a receiver, e.g. to receive a signal for opening a valve on demand (as I pointed out in my previous posting).
EDIT: OK, this seems possible. But is it also possible to connect both, a transmitter AND a receiver?
Thanks in advance regarding the other questions!