Well going to throw my two cents worth in here. If I was you all the first thing I would do is have the businesses agree on how they will operate. If nothing else to document how the different sites or business operates. A simple example is one site backflushes common stock material and another does a push function(pick and issue) - both will work in most ERP systems, but sometimes something like this might cause configuration or software setups that impact the other site. So agree how to operate in some fashion first - maybe they are already have something like this. Depending on support works they might even want to agree how to create parts, PO, etc. There are some larger companies that may have experience with the different ERP's - not really sure.
After agreeing to the above I would bring all three in and have them do the sales pitch. And you can ask pointed questions and everybody can see the various systems. See what might work best for the way all sites agree to do business. I would not pick an implementation partner until you decided on the software to use.
Right now we are in the process of going from 4c4 to existing oracle erp in one case and a mapics(I think) to the same existing oracle erp. The biggest struggle I see us having is how the existing oracle erp is setup. It was configured, modified for the current division it supports - there are functions and features the other 2 sites can use that are not available to us. I believe we should have started with a fresh install of oracle - no RIE anything, just a plain install and agreed how to operate and then convert all 3 into it. Right now we run into basic issues that cause issues like part number naming, item revisioning, and even how text is formatted and printed. Again if I had a say we would not convert into an existing ERP.
My 2 cents worth of opinions for the day.
My latest mantra - make sure you have latest stpapi patches and the latest session object. If on LN then please explore the option of using DAL2 functionality.
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