If I’m understanding you correctly, you might have slightly the wrong idea of how openHAB2 works.
Your ‘panels’, assuming they are only for user interfaces, will do little more than pull content from your openHAB2 server, which in turn pulls data from the various bits of hardware that are actually controlling your home’s devices.
So yes, you need a main server.
(Which can have multiple LAN / WiFi devices if you wish)
A raspberry pi would be the absolute minimum and a $1m mainframe would be a massive waste of computing power. 
If you want your Tablets / touch panels to provide environment data (HVAC etc) then you could consider a POE Android Tablet running HabPanelViewer and report back status to openHAB2.
However, ( Who won the sweepstake on me this time?) I would recommend you look at a multi function user interface like maybe a Velbus Glass panel?
https://www.velbus.eu/images/products/0/vmbelo-pf_red2.jpg
Other brands of hardwired panels are available, but I just can’t think of any right now… 