It's been a long time since I did builders cleans (thankfully) but I'd be wary if I was asked to quote on this basis. Builders cleans are generally three stage (build clean, re-clean and sparkle) and the last of these is usually a light clean on an area which has already had a re-clean and which only needs light dusting and sweeping. It's also usually the cheapest which is why you may be getting asked to quote for this clean. I'd want to know who had done the original clean and why they weren't being asked to complete the job - are they too expensive or haven't they been paid for the original works? Either way, it wouldn't be good news from my perspective.
Also, when I did builders cleans, I had to be CIS registered - no construction registration means no payment. That may have changed of course but it's worth checking out.
Finally, I wouldn't assume that you'll have ready access to water or 240v electricity. Apologies if this is all stuff you know already but it's not straightforward and more than a few companies have incurred severe loss through taking jobs like this on.