We bought our 2009 D40 new and she's just ticked over 355,000km. I think the biggest thing that's saved it from major dramas is watching the coolant temps - I try to keep it below 105C (mostly succeed, but sometimes it's touched 106). I achieve this by using the gears on hills (up and down). Make sure your radiator is in good nick and the fan is also good with no cracks in the plastic particularly at the blade base.
A couple of tips:
1) A Go-Pro is a worthwhile investment but don't forget that a modern mobile phone has a camera that takes awesome pictures.
2) While we have Aldi mobile phone accounts, we also carry a Telstra sim around for those areas that don't have "normal" coverage. That said though, coverage across Australia isn't too bad and sometimes it's actually nice to be "out of reach".
3) A laptop with an external hard drive of at least 1Tb is a big help in transferring/backing up photos, some of which you'll never get again. Plus you can store the Go-Pro footage (and if you're going to get a lot of that, consider 2x 4Tb drives - one to store the data, the other to back up to).
4) Use a tool like WikiCamps to find a good place to stop. I don't always agree with some of the reviews in there - there are some places, for instance, where I really disagree (eg Bingara, Deepwater) but on the whole the app is pretty good to use.
5) Enjoy the trip. It's the sort of thing that you might never get to do again!