Unless you are manufacturing a great deal, carrying a lot of inventory for manufacturing or having problems scheduling materials into your plant, I would avoid the additional cost of an MRP system. Your distribution business can not be managed with an MRP system unless you have a good means of forecasting demands. Your subcontracting business is dependant on getting orders which you probably have no control of. Your engineer to order business requires good forecasting of your standard parts.
Getting my personal caution statements out of the way. There are several large players in MRP/MRPII/ERP Oracle, JD Edwards, Peoplesoft, SAP, IBM. Expect relatively high costs. There are many different smaller packages with varying degrees of functionality. You will have to go through the sales spiel and decide which package suits your business. Capabilities of the different packages change over time so unless someone is reviewing the different packages on an ongoing basis you will have to review yourself and make a decision on your own criteria. Check you cost to benefit numbers and undstand you will have to add people to support whatever package you decide to purchase.