Useful topology makes editing, material assignment and deformation predictable. The right polygon count depends on the shot: a distant prop and a close product image have different needs.
Start with the intended use. Static renders can tolerate structures that are awkward for animation. For deforming objects, inspect edge distribution near joints; for distant objects, prioritize the silhouette.
In Edit Mode, use Merge by Distance only with a deliberate threshold. A large value can join separate pieces. Face Orientation helps find outward surfaces with reversed normals.
Shading is not geometry. Check normals and sharp boundaries before adding more modifiers. A small, controlled bevel often improves a hard-surface object more than subdivision everywhere.
Keep an untouched copy before substantial cleanup. Compare silhouettes and important details rather than judging the result only by its polygon count.