Sandblasting is a good idea but concentrate on the common rust areas..sandblasting all of it will mean a lot of work.

I'd do the following:

Rain gutters
Front lower fenders (guards) inside and out.
Under the dash.
Under the radiator.
Inside and out of the back guards.
Pull out the Tail lights and sandblast/rust treat around there. Same goes for the indicators on the front panel.
Sandblast the sound deadening from areas where body panel sections are joined under the car. Water can condense and sit between the deadening and the metal and slowly rust things away. It's a good idea to fix that. When rust starts getting structural..For safety reasons, the car really should be crushed.

I imagine on a ute, sandblasting and treating the tailgate is pretty important. Especially if it's been cheaply repaired in the past.
Pull out all of the glass and sandblast around the openings, if there's signs of rust.
Tops of the strut towers.
Lower body rails in the engine bay.
The chassis stiffening bracing under the floor.
Sandblast around the front footwells if rust looks like it's there.
For the sills, Pop out the rubber plugs and squirt rust treatment into the holes, drive the car around the block a couple of times to slosh it around..then carefully drill a couple of small holes to let the solution out. You will fill those holes with bog.

Now remember, bog is your friend. Although not structurally great. It's better than using a mig or oxy for flat panel repairs..why? because the heat from oxy promotes rust by flaking off any treatment or plating that's been applied to the inner (unreachable) side of the panel. This is probably why cars are spot welded together, to reduce heat impact on the metal surfaces. Not to mention it's quicker and lighter than other alternatives.

You might not like my "the car should be crushed" statement above, but I saw a friends Mazda R100 after an accident once. The whole front of the car was ripped off. The points where it ripped where completely rusted out. It could almost have happened if he'd jumped on the brakes hard enough. The car was a semi show car, so it looked fine..but the cancer was waiting underneath.

Some food for thought.
Chris