.5mm vs 1mm Oversize Pistons
 

My D24 NA engine block is currently at the machine shop.
They said that I could probably get away with using the old pistons with new rings if I really wanted to (Not inclined to do so) as the cylinders aren't super worn.

My initial thought is to only step up to a .5mm piston (to enable future cylinder work if it wears down again), but is it worth considering a 1mm piston for the extra power/fuel economy?

That agrees with my experience: D24 don't usually suffer appreciable wear in the lower end. Unless run with low quality oil or excessive load before reaching operating temp.

Why go oversize at all if it is not indicated?

Are you asking if 1mm oversize pistons will improve fuel economy?

Best bang for the buck to improve power for a D24 is to D24 +T.

They were leaning towards over sizing, but they added that it's probably "just" passable with old pistons and new rings if I can't find a new set of pistons.

Will oversize .5mm or 1mm have any noticeable increase in fuel economy?
Any additional power is just icing on the cake.

Wouldn't adding a turbo to a D24 NA require the following?
Exhaust manifold, turbo header, , turbo injection pump and injectors, turbo oil pan and oil cooler?
I've also read that adding a turbo to a D24 block can decrease the life of the engine compared to a D24T block?

Negligible effect from just the oversize. Possible slightly less.
You'll see far more positive effect in both economy and power from the new rings/hone and correct thickness head gasket (higher compression).

And piston squirters, intake manifold (possibly?), air filter. Might be able to modify the oil pan.

Supposedly the block is a different alloy, don't know what difference it makes in real life.