Web based IT ecosystem on a shoestring?

I might start working as the IT guy for a real estate startup which really needs a streamlined IT eco system to be succesful. What I would like to have is a web based ERP that manages data about staff, sales, real estates, buyers/sellers, accounting, etc. I also want to be able to create totally separate sites and blogs that can exchange data with the ERP.

I understand that such a system might be complex and espensive, but I've done enough research to know that there are open source ERPs, CMSs, blogs, etc that are quite competent. I don't, however, know if there is a combination of open source product that developers prefer to work with. Do you? If you had to create such a system with of the shelf products, which ones (floss and commercial) would you chose. If you think I'm completely off base, I'm more than willing to hear your suggestions on alternative solutions.

Thank you for your insights.