Alan, that is a major task even for a larger, more established company than yourselves. The risk is massive if there are penalties for not completing the job on time.
Then you add on the issue of employing people whom you would need to trust. Would you be able to get such staff on a short-term basis?
I don't see the paperwork side of H&S as such an issue. It's easy to prepare method statements and risk assessments that are generic but could be tweaked on a laptop when you arrive at site and then printed off.
Then you have the capital outlay of equipment which you may not use again as well as the laptop and printer.
What do you think they are up to?
I wouldn't waste my time thinking what they are up to, I would be more concerned about the alarm bells ringing loudly in my head.
I would definitely walk away - and I'm a risk taker!
Chris