What a surprise. You need lots of cash, technical knowledge and time to fix this problem.
Costs you about a grand minimum just to fix the climate control. More if it has been not working for a while. All the seals would have dried and the coolent would have leaked (evaporated). Have it refilled and it disappears again.
Once it's dried out you've got to replace all rubber bits in the entire system.
Plus valves, vaccum hoses, for the control. If the climate control does not work you've got no proper heat.
Since it is a 728 only and one can possibly find other faults. you better get rid of it for every penny you can get.
Because whatever you get is better than having to pay the junkyard for taking it.
Or spend some more money on it, convert it to LPG, repair the Aircon.
Than you might get £2000-2500 for it. Very attractive car, in particular if you can say it's as cheap to run as a POLO. Half the costs per litre on LPG.