@Amy M3 Most important is to remove everything you don't see. usually the vendors make complex drawing with everything in there. But for instance if you look at a forklift truck, you don't see the engine, how the wheel is constructed etc. You see the body and the wheels, so everything else underneath the body can be removed.
Other solution would be a polygon reducer. In the past I have heard some good stories about polytrans being able to reduce the size of a file. You can specify with what percentage you want to decrease your file and it takes away polygons. So you can define at what level you find it acceptable.
