|Your 500' distance is going to be the killer. 300' is the maximum length you can run cat5 cable without a powered repeater and is about the same effective range for wireless networking with clear line of sight. Q? Would you be able to set up a powered device near the midpoint?|
As for the desktop there are numerous options for wireless nics in either PCI card or USB dongle.
Your other hurdle is the cradlepoint device. As I see it that device can take internet input from either the USB modem which you have or via an inbuilt WAN port. It's only output however is the wireless. It has no client-side LAN ports. Your WirelessN router may be able to be configured as a repeater or perhaps in Client mode as an accesspoint so you would be able to run wired client devices from it. You would need to tell us the make/model if you cannot work out from the manual whether yours has those configuration options.
Goin' Fishin' (Some day)