-48V means it is telco equipment. And it is minus, not plus, because this prevents oxidation and destruction of wires due to electrolysis.
When the manufacturer says you don't need a regulated supply, then you can use an unregulated on. Although a regulated one should not hurt. They should also have specified some limits for the unregulated supply (min and max allowed voltage).
When the manufacturer says it needs to be isolated, then get an isolated one. Which also fixes your earth reference problem "should I reverse the polarity". If the supply is isolated you just need to connect its output correctly. They hopefully have marked the polarity on the equipment correctly. If in doubt, ask them. If still in doubt, open the thing and trace the wiring and circuit.