One idea about buying the camper van is to look at backpacker hostels. When living in the caravan park in Cairns years ago we saw the same vehicle several times, each time with a different owner. Of course they haven't had 'one careful owner' and you could get a lemon but all the ones I saw were sound. Given they don't spend much time in cities they've probably had a pretty easy life.
I'd go the inland way to save distance and avoid the crowds. Years ago some of those roads were lonely, which is good until you break down. Perhaps they are busier now but would still be much less busy than the coast - even with all the eclipse traffic

. It will also be hotter but there are few hills or bends so it's easy on the vehicle. The obvious precaution about breakdowns is to carry plenty of spare water so being stranded is a hassle and not a disaster. I've got a 50l water tank in the troopie but also carry an extra 20l 'just in case'.
We look like we will take about 8 days, so there will be days off (at least one at the lava tubes) and I haven't planned on more than 600km on any day. The longer days are at the start then I can relax and have a few more stops along the way.