Newbie Microsoft 2003 Exchange & Leopard Server

Hi,

I was watching the Sean Collings Leopard Server Essential videos on Lynda and he mentionned this website, I started reading a few topics and the people are very knowledgeable like most of Mac communities.
I want to become an excelent system administrator and I have done some training and read some books but I have never set up a real server.

I need some advice before jumping into my first project, I really appreciate any opinions and advice.

Here’s the background:

A small business with 10 – 15 user presently using a Microsoft Exchange 2003 server.
Main services used are Mail, Calendar, and Contacts.
The company has several Blackberry and iPhone devices which used the Exchange for pushed information.
Remote access is needed, so VPN.
Web hosting of the Company website.
3 – 4 Leopard clients and the rest Windows XP clients.

However, the CEO and Vicepresident are using Macs and iPhones and are very satisfied with Mail, Address Book, and iCal and they are gradually switching the whole company to Apple Products. I had to setup Entourage to be able to sync with iCal so that they can use iCal for the calendars with their assistants, but the CEO is fed up of all the problems Entourage creates and wants a solution. Our only problem is iCal because Mail and Address Book sync perfectly fine with the server.
Snow Leopard was supposed to fix all these problems and finally leave Entourage but S/L is only compatible with Exchange Server 2007 so the CEO has decided to upgrade to the Exchange 2007 Server. he is willing to pay between $4000 and $5000.

I think I can setup something better with Max OS X Server.

So here is the question:

Is it possible to setup a Workgroup Leopard Server to provide shared Calendars with an Exchange 2003 Server providing the rest of the services?

Thank you again I hope I explained enough and I appreciate your answers.