I have no idea about cozy.io, but just to offer another option, I’ve been running Seafile for years and it’s pretty solid piece of hardware. And while it does have other stuff than just file storage/sharing, it’s mostly about just files and nothing else. Android client isn’t the best one around, but gets the job done (background tasks at least on mine tend to freeze now and then), on desktop it just works.

It depends. I’ve ran small websites and other services on a old laptop at home. It can be done. But you need to realize the risks that come with it. If the thing I’m running for fun goes down. someone might be slightly annoyed that the thing isn’t accessible all the time, but it doesn’t harm anyones business. And if someones livelihood is depending on the thing then the stakes are a lot higher and you need to take suitable precautions.

You could of course offload the whole hardware side to amazon/hetzner/microsoft/whoever and run your services on leased hardware which simplifies things a lot, but you still run into a problem where you need to meet more or less arbitary specs for an email server so that Microsoft or Google even accept what you’re sending, you need to have monitoring and staff available to keep things running all the time, plan for backups and other disaster recovery and so on. So it’s “a bit” more than just ‘apt install dovecot postfix apache2’ on a Debian box.

Others have already mentioned about the challenges on the software/management side, but you also need to take into consideration hardware failures, power outages, network outages, acceptable downtime and so on. So, even if you could technically shoehorn all of that into a raspberry pi and run it on a windowsill, and I suppose it would run pretty well, you’ll risk losing all of the data if someone spills some coffee on the thing.

So, if you really insist doing this on your own hardware and maintenance (and want to do it properly), you’d be looking (at least):

• 2 servers for reundancy, preferably 3rd one laying around for a quick swap
• Pretty decent UPS setup, again multiple units for reundancy
• Routers, network hardware, internet uplinks and everything at least duplicated and configured correctly to keep things running
• A separate backup solution, on at least two different physical locations, so a few more servers and their network, power and other stuff taken care of
• Monitoring, alerting system in case of failures, someone being on-call for 24/7

And likely a ton of other stuff I can’t think of right now. So, 10k for hardware, two physical locations and maintenance personnel available all the time. Or you can buy a website hosting (VPS even if you like) for few bucks a month and email service for a 10/month (give or take) and have the services running, backed up and taken care of for far longer than your own hardware lifetime is for a lot cheaper than that hardware alone.