So don't take this as rude, but if none of you have experience running email for a business, you're probably better off contracting that part out.
It's a lot of work to get working, keep working, and is prone to exploding for no particular reason so if this is a business-critical component, it's worth the $20 a month to get it hosted where making your email actually deliver to people's inbox is someone else's problem.
Same for the business website: if it being down is going to cost money, a simple static page like that is hostable for literally free with cloudflare or netlify or any of a couple of other providers, and that's probably what I'd do. (And, frankly, is what I do with a lot of stuff I host.)
As for storing and accessing remote documents, if you pay for gsuite or office365, you'll get that included in the price, so like uh, that might be the best way to go.
I know this is literally not what you asked, but....