Networking - File Sharing - Backup

Small Auditing Partnership Firm. 2 PCs and 2 Laptops connected in a Network. All used for same tasks like MS-Word, MS-Excel, pdf, TALLY(Accounting Software), etc none of the files will cross few MBs. Nothing power hungry use. So, we decided that we don't need a NAS. We decide to save all the files in 1 PC and share via Network to other Pc and Laptops. We build this PC with better spec like R5 3400G, 16GB 3000MHz, 512 GB NVMe SSD. This may look like low end PC but for the works we do this is an overkill. Well, want to save all the files in this 512GB SSD (Believe me we won't even need 100GB a year, so this SSD will last at least 4 years) and share it via the Network. The datas are very important to us. Without it zero work. So, here are my questions,

1. Were our decisions correct?
2. What kind of backup do I need? Should I use cloud storage or external HDD?
3. If cloud then recommend something reliable and cheap as we don't even need 20GB a month. And I don't know how a cloud storage works. If 20GB a month, will I have 20GB of data that I stored in previous month?
4. If cloud, then is there any way to sync periodically data from Network to cloud?
5. Or should I use a HDD in another PC and back up the files to it via the Network itself.

Forgive me I am wrong somewhere.