Are you providing a support contract long term? Are you backed by multiple people in case you’re away and their business is down? I say this more figuratively than specifically you, this could also apply to their internal IT guy who wants to do this.
I’d strongly suggest deferring to a local business IT services company, unless you’re an active partner in the business. They should find a company they are comfortable with and trust, then use the products they recommend and are comfortable with.
You could do something like nextcloud to solve a lot of issues, but I’d still hesitate to recommend on-prem hardware and managing hardware yourself. It really comes down to the business tolerance for outages though, maybe the computers being down for a day or two doesn’t matter.