This really sounds like a task for openHAB. 
I tried some time with Volkszaehler, but I didn’t get warm with it somehow - so if you also posted something on volkszaehler, perhaps the guys over there can help you out!
openHAB isn’t specialiized in powermeters to begin with, so as said, if your only goal is to have some Information on powermeters and nothing more, perhaps volkszaehler is indeed better suited - but I don’t know much on that.
what you get with openHAB on top (I don’t know, if volkszaehler does that, too) is rules and Actions. Meaning you can easily mail/sms/telegram/update a website/… with the readings. or use a persistence and HABPanel to generate cool graphs.
for openHAB:
- Elster A1500 - i don’t know the protocol of that one, but it seems (from pictures) it uses the standard EHZ - interface. There’s a binding for this in the marketplace (SML Reader - how to integrate - #26 by binderth)
- Modbus/S0 - I don’t know modbus, but I know, there’s a binding for it: https://docs.openhab.org/addons/bindings/modbus1/readme.html
- Intergaz - if ist an magnetci pulser, you can use a simple S0-counter (i have onewire with my BK4 gasmeter) https://docs.openhab.org/addons/bindings/onewire1/readme.html
- middlepressure gasmeter - if there’s indeed no interface - you can still Mount an Webcam nearby and take Pictures, which get then analyzed - works pretty well I hear
- watermeter - could be, that that ones got an magnetic pulser also (see above)
hth