Try doing some searches on this site and at http://www.blitzcoder.com/ for visibility, vis, occlusion, portals, frustum, and culling to get a clearer idea of how to go about managing mesh visibility. All that has been discussed many times before and you should find plenty of information on the subject.
As far as positioning objects goes, what you need is a good map/level editor. Try searching the toolbox section of this site ( http://www.blitzbasic.com/toolbox/toolbox.php ) and the showcase at http://www.blitzcoder.com/ ( http://www.blitzcoder.com/cgi-bin/showcase/showcase_frontpage.pl ) . I think the Gile[s] lightmapping application in the toolbox has a good map editor built into it, too.
Maplet in its current form is probably not what you're looking for as it's only designed for creating and lightmapping meshes, not for positioning imported items.
As far as positioning objects goes, what you need is a good map/level editor. Try searching the toolbox section of this site ( http://www.blitzbasic.com/toolbox/toolbox.php ) and the showcase at http://www.blitzcoder.com/ ( http://www.blitzcoder.com/cgi-bin/showcase/showcase_frontpage.pl ) . I think the Gile[s] lightmapping application in the toolbox has a good map editor built into it, too.
Maplet in its current form is probably not what you're looking for as it's only designed for creating and lightmapping meshes, not for positioning imported items.