@raaahbin It's worth at least looking at Odoo as an open source option: https://www.odoo.com/
If your IT folks want to evaluate it, it's simply a matter of spinning up a VPS at somewhere like Hostinger: https://www.hostinger.com/
Then setting up a subdomain with your hosting provider and pointing it at your VPS.
Then installing YunoHost on it: https://yunohost.org/
Then installing LibreERP: https://apps.yunohost.org/app/libreerp
But Odoo (or LibreERP with some paid modules) will do pretty much all you described, plus things like CMS for your website, marketing automation, etc., all on one platform.
It's easy enough that just about any idiot could set it up.
And by any idiot, I mean me.
Here's a website hosted on a test install I set up: https://erp.sadauskas.id.au/
Beyond that, I would also suggest Nextcloud, which is a cloud storage platform (like SkyDrive, Google Drive, DropBox etc): https://nextcloud.com/
It also has a host of apps that can be installed for things like Calendars, cloud document editing, web email client, messaging, etc.
Again, can be hosted by Nextcloud itself, installed from command line, installed on a VPS using YunoHost, or through a managed provider like LibreCloud: https://www.librecloud.host/
Hope that helps?
@crabmusket @ajsadauskas @raaahbin an admirable aspiration, but also a lot of work. Because clients want all those non disclosed things proprietary software does.
But in some contexts FOSS works really well for small businesses. Typically when they have a service provider that is passionate about that choice. :)