The problem with all these modern units, is that they are designed to be fitted in the car, under the dash. So, if they still physically fit into the space of the original heater box, almost everything regarding the inlet and outlet ducts falls in the wrong place. The fact that part of the P6 heat / ventilation system is inbuilt in the bulkhead doesn't help. Of course if you are determined, you can manage, but there is a limit that your efforts will be worth the result.
Unfortunately inside the cabin there is next no space available. This is why almost all installations sacrifise the centre console, with various amounts of success.
I have also seen a Greek 3500 S where the evaporator was installed deep in the passenger footwell, and then it was connected with ducts to the rest of the factory system. It did sacrifice the depth of the passenger footwell (it wouldn't have been comfortable for tall passengers) and the space of the passenger glovebox, but it left the interior looking standard. I cannot comment on the efficacy of the system, but i guess that it would not have been too bad.
My problem with the systems that can only recycle, is that you have to effectively seal the cabin in order to allow them to be effective. This more or less works around town, but at speed you cannot avoid the leaks of hot air into the cabin. Not to mention that it is not a good idea to travel on the motorway without any fresh air in the car.