If you want to do this as part of a commercial venture then I would say buy a copy of AutoCAD (Approx £3000).
Some of the AutoCAD expanded versions might be better. (Try Mech Desktop)
I have heard that TurboCAD can do 3D fairly well for the price. You don't have a member of close family studying at Uni or School doing an engineering or design type course do you? Cheap versions of AutoCAD for them.
DeepC