Ah, the Smokies. Doesn't get much better than that place. As a start, visit the NPS GSMNP site. Lots of info, weather links, and best of all, a downloadable full park map showing the entire trail network (there's alot, over 800 miles!). The map shows you all the potential loops; for a loop 60-80 miles long, I'm thinking AT north from Newfound Gap to Tricorner, down into Big Creek on Camel Gap, up Swallow Fork to Mt. Sterling, then south on the Benton Mackay route and then west on Kephart Prong/Sweet Heifer Creek back to the AT and Newfound Gap.
As for weather, you could have anything from late-spring conditions to deep-winter conditions, maybe all on the same trip. I had a trip about 10 years ago that started in the 60's and finished in the teens with 3 feet of snow! The big things to be prepared for in March are cold rain and mud. Make sure your raingear is solid, and take gaiters. Check out Chris Townsend's article here on hiking in cold rain. Be prepared for winter temps and possibly snow on the AT and anywhere else in the park above 5000 ft. elevation. The big highspots (LeConte, Clingmans, Guyot) can have temps around 0F even in March.
Also, be wary of the bears. This is the highest density of black bears in the East, and they aren't as fearful of humans as in many places. Check trail and shelter closings on the NPS site and at the trailhead before starting. I know Russell Field shelter on the southern AT is closed right now because of aggressive bear activity.
The best book I know of on GSMNP is "Hiking Trails of the Smokies" by the Great Smoky Mountains Natural History Association. It has a mileage/elevation profile and narrative about every trail in the park. Awesome resource.
Have a great time!