Need to do a network inventory.

Hi all,
I was assigned to perform an extensive inventory of our network (150 workst; Windows & some MACs).
Need to find out about hardware, software, licenses, network settings etc.
Budget - up to $400
Due date - next week.

I am looking for a good (the best possible) recommendation for a software that is easy to use and is able to discover / scan the network from a single location without deploying additional software to each workstation.
After doing a three day search I am almost ready to give up - couldn't find the best thing.
Please, only recommend from your personal experience. I already know what GOOGLE offers.

Thank you in advance.