Assuming you're doing a real migration (and not just starting a new AD forest and Exchange organization) you want to keep a good copy of Active Directory around.
If you can bring up a secondary domain controller on even a PC-class machine it'll help matters out. You don't want to lose all your copies of Active Directory or it'll make your life more complicated. This saves you from having to restore Active Directory in the event of a failure.
You can't restore System State backups to unlike hardware so, while I agree with @Vick Vega re: taking full backups (including System State) of your current servers, you can't restore them to other machines easily. Having a secondary domain controller makes this less of an issue (though you still need System State backups for off-site storage of AD backups).
Protecting Exchange from disaster during your recovery process is fairly straightforward. Perform online backups of Exchange and restore them if you need to. If the current server fails you can install a new machine, use the "/disasterrecovery" switch to install Exchange, and restore the online backup to it. There are no concerns about unlike hardware in restoring Exchange and, as long as the only thing on the box is Exchange, you really don't need to backup anything but the Exchange databases.
The "PC-class machine" would have to be running Windows Server. I brought it up because you talked about impending potential hardware failure. I'd bring up a secondary DC quickly if I were concerned about failure of the original DC's hardware. You can go ahead and bring up your W2K8R2 DC as the secondary, too, but I got the feeling that it might take you some time to get there.
/domainprep aren't "high-risk" updates in my experience. Obviously, have good backups before you start (though recovering from failed schema modifications are hard even if you do have good backups).
No matter what hardware you use, get a secondary DC up quickly if you're worried about hardware failure. Losing AD in this process isn't going to make it any easier. That will turn a straightforward migration into an ugly reconstruction.