You need to learn only enough about NT to know how to migrate it to Windows 2000.
Since it sound like you will need to install Exchange 2000, your Win2K experience will help, since Exchange 2000 MUST be installed on a Win2K Server, and you MUST have a running Win2K Active Directory (AD) domain. If you are going to go this far, you might as well migrate your entire domain (unless there are operational issues that might prevent this).
The availability of online courses (or classroom courses for that matter) for WinNT will probbably limited as Win2K has been available for some time.
I know these two organizations offer online courses:
New Horizons (www.newhorizons.com)
Global Knowledge (www.globalknowledge.com)
New Horizons offers only the Microsoft Official Curriculum (MOC) for Microsoft products. You won't find any NT training there since all NT tests (and the corresponding classes) have been retired. Also understand that ANY classes you take that are based on the MOC have the primary goal of preparing you for the corresponding Microsoft certification test (vs. learning what you need to know for your day-to-day job - they aren't necessarily the same thing).
Global Knowledge claims to offer some NT courses from third-party vendors. This might be worth looking at.
Depending on where you are located, you might also take a look at Learning Tree International (www.learningtree.com) if you have the opportunity for taking hands-on courses. Learning Tree courses are NOT based on the MOC (as far as I can tell from the desriptions).
All these organizations offer Exchange and AD training.