convert STL surfaces into solids

We need 3 STL files of machines with several parts converted in IGES or STEP so that solidworks can manage them. We need them as an assembly of the several solids each machine is composed of. The STL includes also all the inner parts of the machine, for this project we need only the outer parts (that means that inner parts that are hidden by the casing can be cancelled, thus reducing the computation power needed for the conversion). Please be careful to convert the surfaces in the different solid that make up the outer casing, so we can apply different txture and material to each solid for rendering purpuses. Each file is about 98 MB. I can send link to one file to selected freelancers, so you can see if the job can be done. Thanks