Discover.
A senior engineer–designer pair spends a week with you. Reading the existing code if there is some. Talking to the users you actually care about. Drawing the system on the whiteboard until the parts that won't work fall out. You leave the week with a written plan, not a sales deck.
You leave with
- A written architecture document, ~10 pages, yours to keep
- A fixed-price roadmap for the build phase with honest tradeoffs
- Red flags called out by name, not buried in a risk register