Welcome to openHAB.
Please note that this forum is not on general Home Automation but on openHAB only. We don’t do general HW consulting and discussion here.
Some general advice:
Automating HVAC is the hardest part in Home Automation. We often get to see post like yours, but only the inexperienced start with that and often need to return and rework their heating setup later on.
Get experience and confidence in openHAB and one or two HW standards first (KNX, ZWave, Zigbee) before proceeding.
I personally would never use proprietary stuff like Homematic but would look for a standards based solution instead. Granted, for thermostats there’s not much else on the market so you have to make compromises. Check out the MAX! system, that’s essentially Homematic for thermostats only and works without a CUL.
For a server, best use a Pi with openHABian, it comes with an option to install homegear to run on the same box if you really decide to go that Homematic route.