The best solution for you in this instance would be Intel's LANDesk Management Suite.
It offers support for the full Windows products and also Inventory and Remote Control for Macintosh OS.
It is a fully function management tool for your organisation.
It integrates fully with a number of leading helpdesk products and is fully web-enabled.
I admit a bias here as we are a Deployment VAR, so no hiding there. We have installed in numerous environments similar to your own with great success. We also deal in Helpdesk product too so can offer a full solution.
We have offices in US and Europe, so please go to our Website: http://www.networkd.com and it will list the contact information to progress further if it is something you are still pursuing.
Regards.