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.
We are OEM company that produces to the forecast. WE have purchassed Mapics and have several implementation problems. Software right out of the box do not cover all your needs and the modules I believe are written by people with limited manufacturing experience.I am hearing good things about JobShop ERP system.Stay away from the big packages that are more expensive and you will end up with consultants establishing residence in your shop like it is happening to us.Do not rush the selection process,have a committee that will make recommendations and if you can afford it hire an independent consultant twith ERP experience to help you make a choice, that will save all dwnstream problems.
You might try looking at Metalsoft of Irvine, California. They have a number of software applications for sheetmetal fabrication and material management.