In my experience depending on the model, it can be between hard and very hard. Blenders remesh will produce something reasonable but with a gazillion polygons. Guided approaches are a lot better but are labour intensive as the user is effectively tracing over the source model.
Retopology will destroy the old uv map, so a new map will need to be made. Following that, the old textures will need to be baked onto this map.
I have spent easily as much as a day cleaning up (I.e. re-mqking) 3d scans.
You can download the models from Hugging Face.