The KPD in particular got banned for wanting to topple the government by force, also, they took their orders from Stalin. Being a communist as such is far from anti-constitutional, the German constitution was specifically written to be compatible, but it's going to have to be democratic market socialism. What you want to do is heavily lean on Article 14(2): "Property entails obligations. Its use shall also serve the public good." You can expropriate means of production by mere preference, without having to show that it's for the public good.