1. Document it, then contain it
Record what's wrong, limit further damage where it's safe to do so, and pull together any history on the windows.
2. Look at the whole house and lot
Flashing, sill condition, the frame material, glass package, trim restoration, and solar exposure all shape the result about as much as the window unit itself does.
3. Get the scope in writing
Nail down preparation, protection, exact materials, access needs, who pulls permits, testing, cleanup, finish restoration, exclusions, and how change orders get priced.
4. Weigh the reasoning, not just the number
Check credentials and compare why each provider scoped the job the way they did — the total by itself doesn't tell the whole story.
5. Close it out the right way
Before you sign off, confirm testing happened, the site is cleaned up, documentation and warranties are in hand, and you know what maintenance is yours from here.