New truck build Australia, suggestions and recommendations.

What license do you have? how much you want to spend? manual or auto? This may all determine age, then with age comes past or present, neglected or active maintenance.
Generally rubber or rubberised plastic doesn't like to sit, seals from engine, gear box, wheels, diffs, hoses, windscreens to tyres doesn't do it well.
Diesel doesn't really like to sit around either, full tank to stop condensation is best, but its a lot to go off or have paid for to stop condensation. Little in the tank allows for condensation and perhaps rust to make a start, but no real lose to drain and change. I'm talking if it sits for 18+months, but generally I think diesel starts to go off after 6 to 12 months.

Its probably best overall to start your little house and take it for an ice cream by the sea every +month to two.

If you Solar it either actively use the trucks batteries as part of the system, but don't run them below 12.1V for a 12volt system. Double that for a 24V system. Or disconnect them once thay are charged up from the run around the block.

Trucksales.com.au is full of old flat beds, and $5k can only buy so much. But if you want to one day travel with your little house, as a general rule it takes a highway running truck about 15 000 hours to clock 1 000 000km, often they are then traded in. Maybe a refurbished agitator for $60-70K with 280K km and 14000 hrs might be a good start. 10K to freshen up the motor and it should see you well for a while.

 

Imo you are taking the wrong approach. House on wheels and a sensible vehicle to pull it.