Most overruns start with copying every habit of the old system.
ERP selection usually begins with a requirements document that maps every current process to a feature. That produces the biggest single cause of overruns: trying to reproduce every quirk of the old system in the new one.
Separate "the business needs this" from "we have always done it this way." A three-step approval chain may be a real control, or a workaround for a limit in a ten-year-old system.
Test vendors on your messiest process, not your cleanest. Every ERP handles a simple invoice-to-payment flow. Fewer handle your multi-entity, multi-currency case.
Budget for change management, not just software. Getting a finance team that has used one process for eight years to adopt another is where projects stall.
Hold back on customization in year one. A close-enough standard process adopted now and refined later usually beats a heavily customized system built to keep old habits.
Ask what happens when something breaks at month-end close. Vendor response time during a close matters more than almost any feature on the list.
The organizations that get value are rarely the ones that copied the old processes most faithfully. They asked which processes still deserved to exist.