Welcome to the Forum. Buying a D2 is a tricky business, but the key is really the chassis. If you bought one with a genuinely sound chassis (or at least
one that can be decently repaired with welding or a part-chassis replacement) then pretty much everything else is economically viable. If the chassis is
shot then unless everything else is superb and justifies spending more on replacing the chassis than the car is worth, then ultimately it is just a parts car.
There are some real BCU/electrical gurus on the site, so post your Alarm issue on the Disco forum and someone will be along with sound advice.
For starters - might be the receiver in the roof lining - plug and play replacement. EKA could be wrong as they can be changed and if the original was too difficult to input it might well have been - Nanocom or Foxwell NT530 will read/change it for you (Hawkeye will not). Could be a faulty BCU, much more complex, but they can be sent away and fixed.