You have to be more specific as RMU[Ring Main Unit] is a switchboard and in my opinion one needs Voltage 6,24,36 kV or else ,rated current, number of switches, cable size and so on.
The breakers also may be Air, SF6 or Vacuum insulated. The manufacturer may be Japanese, Chinese or European [as ABB] or else.
I think better you contact a manufacturer in order to find what will be required.
You can see, for instance, this:
1. The rated voltage at the input.
2. The minimum insulation level at the input required by the local authority, who may in the future increase the voltage at your medium voltage ring and hence would expect the MV switchgear to withstand it. They do not care about the transformer since usually that's your property/worry.
3. You need the value of the short circuit at the input. Typical values are 16, 25 and 33kA
4. You need the sum of the power of your two outgoings.
5. You also need to address the protection requirements.
