3D Model Refinement for Custom Figurine

I have a colored 3D model that needs professional touch-up work to make it fully print-ready. This is for a custom figurine production, so precision and attention to detail are critical.
Specific Requirements:

Model Merging: Combine separate head and body models into a single, seamless mesh
Texture Refinement: Fix minor texture issues and ensure clean, print-ready surfaces
Watertightness: Ensure the model is completely manifold with no holes, gaps, or non-manifold edges
Wall Thickness: Set minimum wall thickness to 1mm or higher throughout the entire model for structural integrity during printing
Print-Ready Export: Final delivery should be optimized for 3D printing (STL/OBJ format)

Required Tools/Software:

Blender (primary)
ZBrush (if needed for sculpting fixes)
Materialise Magics (for final print validation - preferred but not mandatory)

Deliverables:

Print-ready 3D model file (OBJ, MTL, PNG)
Original Blender project file (.blend)
Brief report confirming watertightness and wall thickness validation