What to look for when buying an engine.

Car vs truck doesn't matter too much aside from accessory positions and intake. Otherwise they are effectively the same. You can swap parts freely between them, though I understand you might have a harder time finding parts to swap. Typically the truck engine accessories are easier to work with and require less potential modifications (no frame trimming for alternator, more standard water pump inlet and outlet), but they don't look as nice and the intake inlet is pretty much right on top of the engine. The car intake mounts at the front, so it's a little nicer looking, but with the tradeoff of the accessories being in different places.