I've run a few MRP and ERP transitions and in my experience the seeds of shit are planted very early on if you don't get on top of the following:-
1 the client has very well documented procedures (this is usually true)
2 the client mostly thinks its procedures are followed - they mostly are, but the non-compliances and workarounds will kill you if you don't pick them up now.
3 a lot of stuff is processed off-system (usually in Excel and often using undocumented-macros written by a bloke who left 3 years ago and now lives in France) mostly driven by management desire for pretty colours in reports or because one department is too important to be bothered with petty details like understanding and using ERP.
4 Data integrity will be dodgy; people will have made imaginative use of the data structures for stuff they weren't designed for - like using customer address fields for CRM information because the ERP doesn't have a CRM module or using a foreign currency field for recording special tax/pricing agreements or including legally-required allergy information in a notes field that appears on your screen but isn't visible to people with different creds to you.
Once all this is understood (it can take many weeks) you can have a go at defining user requirements with the users. The management & the department that's too important to use ERP will kick up a fuss about their custom stuff being vital and company destroying if anything is changed and they haven't got time to learn how to do it differently because they've got a proper job to do. Then you spend a day with the ERP sales team and get a ROM** quote for the likely ERP customization and maintenance deltas, at which point everyone gets on board with the fact they'll find a way to make the standard product work (they are lying, by the way) or they won't be having a pay rise for ten years.
Having got all that done - you can start thinking about planning a procurement and a transition.
**it's getting harder to get ROM quotes for customization cos the suppliers treat them as real prices they could be held to. I think that before long the big MRP/ERP suppliers won't be offering custom products - it's not worth the hassle for them, even at eye-watering prices.