You must have the PWR leads, the reset and HDD activity are nice to have. Visit a local computer repair shop and ask for a scrap front panel. You can use the wires in the scrap panel to splice to the Dell case wires and then use those connectors to attach to the motherboard's front panelpins.
+1 with an exception
If you can scavenge a set of cables there's no need to splice anything, just remove the switch and leds from the Dell holder and replace them