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2. Retro fit in advance by providing ATS with quick connect permanently.
I'd vote #2, but this sets up an arragement and a future contract agreement with local generator distributor that might be beyond the scope of your work.
You have to do all the load calculations, assure that the electricial argreement is ready and counted for and present it it to the owner what you find out.
Price of ATS, work on service and circuits involved, additional scope of work (future) if required training/yearly.
You or them could then deal with the local dist. I'm sure there might be some variables but the jest of it is correct.