Garden Wall Construction How Can I Avoid A Cowboy Builder For A Garden Construction Project?

How can I avoid a cowboy builder for a garden construction project? - garden wall construction

I am planning a construction project with a retaining wall in garden and terrace. I have reduced the number of offers from manufacturers, and some who (can afford about £ 2K). All are reasonable reputation in the field.

Has anyone tips on how I can ensure that:
1) The manufacturer shall within budget
2) I have some protection if it fails.

If it involved a contract? I have also heard that many clients ask for money in front for hardware, I expect, and if so, what is the best way to pay for it, etc?

Thank you in advance for any advice.