Any luck tracing the faults? What about the wiring, is it there?
I've had a squint and... Service Box makes no distinction between the heater box for cars with air conditioning and cars with climate control (or "regulation aircon/temp"

), however I'm pretty sure your controls are worked with Bowden cables (think bicycle brake cables) but the climate control must have ickle motors on everything, so I'm not sure if it's a case of ripping off the motors from one in the breaker's and fitting them to your heater box (which SB says are all the same for D8s, except the HDi) or you'll need to change the whole damn thing.
Additionally, you'll need lots of other stuff while you're there:

(apologies for SB screenshot, the coupe parts catalogue seems to have turned into a site about bicycles

)
Note that some of these parts will be on yours already and some you'll have to take off & stick in the bin...
As I said before, if the wiring is there then you might be in with a chance, though personally I wouldn't fancy it.
As for the cruise, it would appear that Pug didn't fit it to any D8 diesel, most of the parts are generic (ecu, brake switch, clutch switch, power relay, vacuum pump, bracket and associated plumbing, control lever) but the vacuum servo and control cables are engine-specific, it only lists the 1.8, 2.0 and V6. I suppose you could try the actuator from the 2.0 (XU10J4R) because it's a cable but then I've no idea if the ecu would get any speed information from the gearbox (although as yours is an ML5T it might 'cos the V6 has the same 'box). It would probably be easier to fit an after-market system (though they don't come cheap unfortunately)
Sorry if this isn't the sort of news you were hoping for
