I think there might be a misunderstanding here of how cloud services work? You can’t have email without it being cloud-based, because the whole point of email is that it lets people from anywhere send another person email via the internet. The “cloud” is just another term for “on the internet”; email that’s NOT on the internet is worthless. What you’re describing might be suitable for some sort of intra-company messaging system, but not for a customer-facing email service.
While hypothetically you could run your own internet-facing email server from your rural barn, you will run into many, many issues maintaining it, ensuring that emails are received (and your responses are delivered). That’s not your fault, no matter what you do… it’s more that decades of spam have required the creation of many email-specific security policies that the big email providers teamed up to create, for the explicit reason of making it harder for fly-by-night spammers on random email servers to deliver mail to their customers. This means that to Gmail, Microsoft, Apple, Yahoo, etc., your homebrewed email server appears just like another spammer, and will be categorized as such, unless you configure everything correctly and stay on top of all the latest security developments every few weeks. That’s assuming you can even keep it online consistently from your rural location; if you can’t, other email servers will error out when they try to reach your server, and tell their users that they’re having trouble. And your customers would probably just move on to the beet farm the next town over.
So going with a standard email provider not only means it’ll be far cheaper (in time, if not money) than rolling your own, it’ll also ensure your customers will be able to send you messages while you’re asleep at the barn.
You think you want self-hosted email, but I think what would work better for you is actually just standard email that works while you’re offline, where you can read and reply to downloaded messages while offline (downloaded the last time you were connected), save your replies, and then send them all at once when you do connect to the internet – whether through dial-up or when you’re in town once a week for the market or whatnot. Fortunately, because of the way email evolved (from the pre-dialup days), this is actually the default behavior for most email programs, from the one on your phone to Microsoft Outlook to even Gmail (with offline mode enabled). Even Microsoft Office, the “cloud” version (Office 365) actually downloads the programs to your computer, only requiring an internet connection for a license check once a month. Dial-up is more than adequate for that.
In other words, all you need is a bog-standard PC, Mac, Chromebook, or phone. Email will work fine by dial-up, especially if you turn off image auto-loading. What will likely slow you down is all the app and operating system updates that will inevitably come, and for that you might just want to hop into town once a week where you can get fast internet, and/or explore satellite/cellular/Starlink/WiMax/rural internet from where you are.
For your (minimal) purposes, a 10-year-old laptop or a smartwatch would have more than the computing resources you need. The processor speed is not your constraint here, technical expertise and stable internet connectivity is. Seriously, there is no reason (given what you’ve stated) to run your own servers… it would be all pain and zero gain.
If you were talking in hypotheticals and there is actually some other reason to run your own business intranet and small biz servers, can you be more specific?