Wi-fi, Bluetooth, NFC Consultation

We feel the following can be done by Wifi, Bluetooth or some NFC. We do not want to use RFID for certain reasons. However, if you feel there is another technology that may be employed, please advise. The following discussion assumes Wifi.

We have a need to read the presence of various field devices (all supporting Wifi and Bluetooth) connected to an access-point, router or gateway (Gateway). Our PC running Windows app will be connected to the same AP, router or gateway also. Our Windows app (MS Vis Std app) needs to ‘know’ when any of these field devices are in vicinity of the gateway. The field devices will be constantly moving. The vicinity area can be defined by you.
We also like to get the RSSI for each device once ‘inside’ the vicinity area. If some custom development using OpenWRT type gateway system can be done, it will be great, however we are open to any of your ideas. Any other solution suggested by you can be considered, as long as a off the shelf gateway HW will support it.
Please submit your best price for this solution. Ignore the budgets specified here.
Once agreed with the concept, next phase will be development if needed.