Hey, great questions.
you'll need an addon to export the model as .obj
.obj exporting is included in Blender, so no addon is needed.
When sculpting a 3d print, is it necessary to retopologize the model or will it print perfectly fine as it is. Also lets say you want to print a weapon and a weapon stand, can you have them in the same file next to each other with good distance or do you have to send them as separate files. Finally, I want to know do all objects of a model (e.g. a hand holding a sword) need to be one mesh or can they be seperate objects but touching.
It totally depends on the model. The three things you'll need to be careful of is the wall thickness, detail size, and whether or not your mesh is manifold.
The first two are addressed in these videos: https://cgcookie.com/lesson/object-scale-wall-thickness/
Having a manifold mesh means that all pieces are one solid, connected object. So in your example, the hand and the sword would need to be connected if you want them to be printed at the same time. If you want them to be separate pieces, you'll need to print two separate .obj files.