Have a look at Odoo. It is an open source ERP and CRM platform that can be completely tailored to your needs, with either freely available apps or by developing your own.

We are currently working with a company in the sheet metal business that uses it and I'm extremely impressed with it.

My only concern with projects like Odoo is that open source ERP seems to die off fairly regularly. How robust of a project do you think Odoo is? We'd like to be able to run it for a long while...

Hey man. Added the Odoo project to chart.ly to get a quick look at the overall health of the project.

Check it out here: https://chart.ly/github-dashboard/odoo/odoo

From the Commits by Month chart, it appears that it has been active for about 12 years and it still fairly active.

Doesn't look like it is going to die anytime soon based on that data.

As described I'm fairly new to odoo myself. Although I believe it has been going for over 10 years and is currently at version 11. So as far as opensource solutions go, this seems like a solid choice.

Also, I believe the cooperation that maintains the project also offers a paid enterprise version that offers mainly a more complete accounting module and handles the migration to any future versions.