Make use of the wagon's "secret compartments" to maximize space and storage:

Under the rear seats
Under the cargo area
Inside the spare wheel cover (bring extra lug bolts and good extendable lug wrench w/ proper size 17mm)
Inside the first aid cover

If you plan to sleep in the car, you can remove the other half of the rear seats to maximize space also. This also saves weight, which is replaceable with parts

IIRC the roof bars can hold 100kg from factory. If you plan to carry additional weight you need to reinforce it or use a supplementary roof rack system instead.

Study the weak points of potential break-away components and avoid touching those or make sure they are in good order. Or create a failsafe mechanism you can rely on to prevent them getting damaged. The radiator nipple going to the expansion tank, for example, is notorious for a handful of headaches where owners leaned on the said component and broke off the part, requiring a repair or a new radiator.

Before heading off, check the battery condition or replenish the water (if necessary). Bring some distilled water on your trip just in case and label it accordingly (you don't want to mix it up with potable water too).

In case you'll be driving into inclement weather (rain, etc), I would just be on the safe side and replace the wiper stalk and/or relay. I just replaced the said switch on my 300D because level I and II wiper function stopped. I thought it was the relay too, but now I've got a brand new wiper relay in case it fails also.

Remove what you plan to not use to save weight and keep things simple.