You’re right that Motorola owns Vertex Standard. I’m not familiar with too many manufacturers other than those two that offer products that support the DMR standard. You might check around to see what Hytera offers. I’m not familiar with their product line, but I do know they support DMR. Let us know what you decide to go with!
Why not borrow the technology already available from the amateur radio community and build yourself a repeater controller that can be hooked up to a VOIP connection.
It shouldn’t be too hard to build a system that uses the internet and is not connected to any other system - due to the fact that public service - you don’t want to let others have access that are not permitted to operate in that service.