What's a realistic rebuild scope for a used 6.0L before I can trust it?

It depends on its mileage and what you find, but Speedway's LQ4 build guide lays out the usual suspects on a high-mileage iron 6.0:

• Piston rings: they commonly wear, causing compression loss and oil consumption. Replace them if compression is low or it burns oil.
• Pistons and overbore: worn cylinders may need boring back into spec, which means oversize pistons.
• Connecting rods: replace them only in extreme wear, or if you want stronger parts for power.
• Valvetrain: stronger valve springs and pushrods if you add a cam.

How much power the stock parts take: Speedway puts the stock LQ4 rotating assembly (cast pistons, powdered-metal rods, cast crank) at about 500–550 crank hp with a good tune. With boost or nitrous, many builders keep it around 450–500 rear-wheel hp for long-term reliability. The iron block itself is strong. Past that, plan forged pistons and rods.

A compression or leakdown test before you tear it down tells you whether you're looking at rings and bearings or a full machine-shop job.