You'll want to ensure that your existing Active Directory and Exchange environment is already healthy. If it's not, definitely clean that up before any migration. You're on the right track with everything else. Transfer DHCP to the new server once it's a member and a domain controller. Installing and migrating Exchange isn't something that you'll be doing, so that's a huge load off your shoulders.
However, I didn't see you mention two important things:
- Transfer the 5 FSMO roles from the SBS machine to the new machine
- dcpromo the SBS 2008 machine to gracefully remove it from the network. Then unplug it. Never just ditch a domain controller. Bad things happen. Baaaad things.
I also didn't see you mention that you'll be using a second domain controller. I would strongly advise that you do, even if it's just a token $400 SuperMicro to have in case something bad happens to the primary.
For more information, check out this blog post about migrating SBS 2003 to Server 2008 R2 (however, the principles remain the same)
You may want to consider some of the products that are featured at www.SBSMigration.com. They've been around for quite a while and will be quite a benefit to you. (Note, I have nothing to do with them and do not profit from recommending them) However I believe that their products are tailored to those who are transferring the gutsier parts such as Exchange and SharePoint.
To check the health of AD you'll want to run an array of diagnostics. Event Viewer is definitely a part of that. Search for all errors on the box in various categories and see what comes back. Other tools are used as well like dcdiag.exe with various /test variants, netdiag.exe and repadming with various switches. Take a look at this MSMVP blog and this ITToolbox entry for more detailed information. I would also run the SBS BPA.
A word concerning licensing
When You move away from SBS, you will be introduced into the frightening world of fractured Microsoft CALs. Sure, SBS has standard and Enterprise CALs. However, your use of Exchange, SharePoint and SQL Server are unified in SBS CALs (which is why they're so expensive). When you start migrating to the non-SBS world, you now have to factor in the cost of Windows Server CALs, Exchange CALs (standard and enterprise), SharePoint CALs, SQL Server CALs, SCCM CALs, SCOM CALs... on and on and on.
Fortunately for your organization (I'm assuming your church has the proper governmental non-profit status) you will get some awesome deals on licensing. I've worked with non-profits and been pleasantly surprised at the discounts that we could get.