I like to do the same. Do you need the Tecbase Mbus 10 RS232-MBus gateway here? LM5 is specified to handle MBUS, so you still require to use another gateway to interface the MBUS master to the RS232 port on the LM5...?
The meter itself is specified as MBUS master and has a RJ45 port.
If you must have a gateway it may even be easier and more flexible to get a MBUS to Modbus TCP gateway although it will be more expensive?
I'm not sure that it will work. Normal M-bus is master-slave where gateway is a master (including power supply) and each meter is a slave. Your meter only uses M-bus physical layer. The meter is sending data over M-bus periodically. This means that you can connect any number of receivers but only one meter to a single bus. In normal M-bus you need to request data from meters so several meters can share a single bus.