Best not to overthink it - The sales clerk is trained to ask for this stuff.
Luckily most times I encounter this I just tell them no I don’t have a phone number with them & continue checkout like normal. Sometimes that means not getting a sale price on something but usually I avoid those type of member-specific sales anyway.
And worst case - Just make something up. At Best Buy a sales rep absolutely refused to sell me something from the mobile dept without my info. Which didn’t make sense because earlier I had bought something at that same Best Buy with a different rep & that rep took my order without my info no problem (she said she had to enter a phone number but just entered Best Buy’s).
Yet this particular sales rep refused to proceed without info, so yeah he got an entire fictional name/address/phone/email on the spot.
Not sure what sort of uptime/reliability your friends are expecting out of a self hosted solution but for something like that you wouldn’t need much processing power, even a Raspberry Pi can host a simple website. Not sure what to recommend offhand but there are definitely vendors in that space that sell simple DIY “contact us” form software, or I guess if you wanted to roll your own that’s an option too. I’d be more concerned about keeping it locked down/secure.
Keep in mind for the internet your friends would likely need business class internet with multiple static IPs so you can give your little DIY box its own public IP address. Many (most?) residential internet service providers do not allow self hosting websites on their network and they’d be dynamic IP anyway though you could work around that somewhat with dynamic DNS since you’re going to need to purchase a domain name and point it to somewhere anyway.
Like others said you really don’t want to go that route unless you’re well versed in that area. It would be annoying for a business especially a new one, those emails will likely keep going into other provider’s spam folders for a good period of time. All the big mainstream email providers are notorious for not trusting new email domains / new IP addresses.
Seems easier to just go to Google Workspace / Microsoft 365 / whatever other provider you like to use, presumably the business has a business use case for reliable email among other things.
Bonus: Those cloud services can easily host simple contact forms for you so maybe that’s your all in one solution. Look into Google Forms and similar.
That sounds like the above commercial cloud solutions again :)
But sure technically you could go through the extra step hosting that yourself. Depends on how the business wants to use/access this stuff, it’s really a question for them. Could be as simple as a Windows server with RDP (if they’re Windows people & just want to log into something “windows” to browse/open files) or maybe multi-user Linux with VNC (the geeks might like, maybe not so much the general Windows/Mac users). Or if you’re trying to do something web oriented maybe something like Nextcloud if you want to do all this in a web browser.
You should triple check what exactly they are expecting when it comes to remote access documents… you really don’t want to spend the time setting up something that they totally weren’t expecting and end up hating.