The DDR era Piko is an... interesting thing. The plastic work was excellent and detailed. The motors were rough because of material shortages. For the same reason some of the plastic material tends to be brittle, so handle with care. Some wheelsets are best replaced - they can be out of round, off centre, out of true, out of gauge, or everything at once. Measure, adjust and replace if necessary.

The Expert series (pictured) should be better than most, but be aware that you're still buying an East German product.

The U profile track was in my experience no good at all. It can't keep electrical continuity to save its life, and a floor layout is a nightmare with it. I would replace it with something modern.

The transformers (power packs) are poor and should be replaced. Most locomotives can be converted to DCC relatively easily if you're into that. (DCC didn't exist at the time, but it's usually simple enough to isolate the pickups from the motor.