If you're doing it as a business. Stick with distributions that are certified Enterprise-class. (Redhat (Rocky Linux) SUSE, Ubuntu Server, etc) Yes, you can use others, but bleeding edge and un-fully tested distributions can lead to real headaches. This is a business, use a Business class operating system.
As for other software you need. Well, first things first. If you don't know, hire someone who does. Attempting to *wing* creating a cloud storage company will leave you in deep $%^& when something goes south and it WILL go south. Especially if you don't know what you're doing.
If you want to do it at the least expensive. But know enough technically I would suggest Debian. Redhat or a clone of it wiould be good also. But stay away from cutting edge and rolling release models. You want stability. If you are new to Linux then you will need to consider hiring someone who isn't. Because if your doing it as a business. You will need that expertise. You may also want to get a lawyer who is familiar with cloud contracts. So you cover yourself. Whenever you run it as a business your placing yourself and maybe others in a liable situation, should things go wrong. It's much more involved than you would think. If your offering it as a service for free you still need to consider some of these things. Just advice. Good luck.
Debian with NextCloud software. I used it in a university setting for a long time before they switched to Google for everything, including google drive. Google drive used to be nice and easy, but now it seems to be to complicated for simple needs.
NextCloud has client software for MacOS, Windows, Linux, and smartphones.