You're going to get a lot of nice features from Active Directory that you're not getting with OpenLDAP. Chief among them being both single-sign on (i.e. one user account that works on all client and server computers), and Group Policy.
I love open source software, but until Samba 4 matures, Active Directory provides the very best administrative experience with Windows 2000 and newer client computers.
Without using third-party software there is no standards-based LDAP authentication with Windows XP clients. Read my answer here re: Kerberos integration with Windows XP-- the experience using OpenLDAP will be very similiar (except that you will need third-party software like pGINA up front to make LDAP authentication work): http://serverfault.com/questions/33564/how-to-get-windows-xp-to-authenticate-against-kerberos-or-heimdal/33571#33571
Whether or not to go with Windows Small Business Server depends on what you want to spend (initial cost and cost of client access licenses for SBS is more than "plain vanilla" Windows) and whether or not you will get value out of the additional "features". I prefer to think of Windows SBS as an inexpensive Windows and Exchange bundle (with an overly complicated setup and cruddy admin tools that I never use.) I tend to administer Windows SBS like a "normal" Windows and Exchange Server machine, and it works very well as such.
A Windows Server with Active Directory, Microsoft DHCP/DNS, WSUS (to provide updates to client computers), and some Group Policy objects to handle configuring user / computer environments and installing software will lighten your administrative load tremendously and make adding future computers easy. Exchange isn't that difficult to get up and running (the biggest problems being related to getting your mail to flow to it from the Internet-- so many people don't seem to understand how DNS and SMTP work together).
Assuming your install is performed by someone who knows what they're doing, and that you treat everything well after-the-fact it will run fine for you w/o a lot of administrative headache. I write off people who bemoan the unreliabilty of Windows and Exchange, because typically they are having problems because they either (a) used inferior hardware and are paying the price in the long run, or (b) are not competent to administer the software. I have Windows SBS installations going all the way back to the version 4.0 time-frame that are running fine years after the installation-- you can have one, too.
If you don't have any experience with these products, I'd recommend working with a reputable consultant to perform the install and get you started with being self-sufficient on administration. I'd recommend a good book if I knew one, but I've been fairly displeased with nearly all of them that I've read (they all seem to be lacking in real-life examples and case studies, typically).
There are plenty of consultants that can get you off the ground inexpensively (the setup you're talking about, assuming that you're going to do the "bulk" work yourself, feels like about a day and a half to two days for a basic Windows and Exchange install, to me) and can help you "learn the ropes". The majority of labor is going to go into migrating your existing user environments (migrating their existing documents and profiles into their new AD account's roaming user profile and redirected "My Docuemnts" folders, etc) if you choose to do that. (I would, just because it will make the users happier and more productive in the long-run.)
You should plan on some kind of backup device and backup management software, a server computer with redundant disks (minimum RAID-1), and some kind of power protection (UPS). I would expect, with a low-end server, lic