Sub-meter water at apartment building

[Paul Shannon]

I'm purchasing a 40 unit apartment building (2 buildings of 20 units each).  Each building has only one water meter.  The current owner pays all water, which runs at $1500-2000/mo.  RUBS for gas, all separately metered for electric.  I'm considering two options to handle the water, which I've been told we can't do RUBS with:

1.  Charge the tenants a flat rate for sewer/water/trash that would mostly cover my costs.  This is simple to implement on units that are vacant or turning over, as we intend to update them, add-value, and will have a blank slate.  More difficult with existing leases, which we will likely leave alone at the onset.  

2.  Sub-metering....If anyone has experience with a product that's worked for them, I'd love to hear about it.  I've heard there are sub-meter devices you can hook on to the cold water line coming into the apartment that can monitor that individual units usage.  I like this idea b/c I could cover all my water charges.  Second, if there is a toilet running, leak etc., we could deduct pretty easily where its coming from.  

Would be curious on product recommendations, as well as general recommendations as to how others have handled this situation.