All things selfhosting and homelab related Resources: - https://github.com/awesome-selfhosted/awesome-selfhosted - https://github.com/awesome-foss/awesome-sysadmin
I would take a look at ERPNext, the pricing page shows $0 for the open-source version with all features: https://erpnext.com/pricing
As mentioned by others, you'll need someone to set it up properly and maintain the software as it will likely become a core part of your business if utilized properly
@iocase With 20 employees, you'll probably want to look at open source solutions unless you have a significant budget available. To get all your employees on Microsoft Dynamics Nav, for instance, would run about $8000 a month.
The drawback of open source solutions is that you'll likely need a dedicated employee to keep the system running and updated, and there's no higher levels of paid support option if they run into problems.
Here's a pretty good article on using Quickbooks as a core and then attaching open source Inventory Management and CRM integrations that you may find useful.
@iocase If you need something simple (inventory, billing, sales) try Dolibarr. If you need something more complete, try something like Tryton (Odoo 8 fork).
I do not reccommend Odoo, as the community version has limitations and you need a subscription un orden to use all the modules.