Land Rover would have said that as they saw "shuttle valve electrical failure" on their diagnostic machine, and LOTS of dealers replace the whole module when just new switches (about £30) "can" fix it, in some cases, probably 50% atleast, new switches "wont" fix it.
You could try new switches, and go from there, a good brake fluid flush can also help to......
Also do your front brakes squeal when braking ? if they do that can cause it aswell.
There is even a "half" kit from Land Rover that retails at about £400, It includes the electrical valves and the switches (the Pump NEVER fails)...... That will DEF fix it, and is a pretty simple DIY job to rebuild your own modulator, but Land Rover (or 90% of dealers) would not get involved in "splitting" the modulator down and rebuilding it, its just to much hassle, so they sting ya for a new one.......
It is ok to drive it with the lights on, you will most likely find that even with them on the ABS will still work, The TC and HDC DEF wont, but not "really" needed for everyday use (unless it snows)
Take the vehicle somewhere with gravel, or wet grass and try an emergency stop, I would suspect the ABS will still function, if it does, then the shuttle valves is your issue.
I would strongly advise against a used one as there no saying it won't have the same issues as yours, I would buy a refurbed one from here:- a little cheaper than Land Rover
Land Rover Discovery ABS Pump/Module ECU WABCO on eBay (end time 23-Nov-09 13:15:33 GMT)
2 years unlimited mileage warranty, better than you'll get even with a brand spanker.
Always put your Location in your profile, someone "near" to you maybe able to help