I'm a huge fan of building the rack at home base, updating the kit and testing to ensure it works on-site. Communication with the customer about your process is a simple way to ensure your customer doesn't believe they're getting second-hand equipment.
If you're facing issues with customers about this, take a look at how your process is being communicated, sort that and it's super streamlined and reduces the risk of stuff-ups on-site dramatically.
This industry is not getting cheaper and cheaper, we're simply convincing ourselves that we have to push the cheapest option to customers. #makesonosgreatagain