March is really unpredictable weather wise in all of NC. Generally it's a very nice month and outside of the mts we start to see green on the trees in March and many flowers, but you just can't guarantee good weather. I was googling just now and came across news articles about the snow we had in late April last year in the mountains (wcnc.com/news/…). I had forgotten about that. We were up in the mts about April 1 and it was very nice, but we weren't there in late April. In the middle of the state (the piedmont) where I live we can have 85ºF/30ºC in March, or more typically it would be in the 60sF/70sF (16-23C).
If you really want to pick a peak time to come to the mts I would say either May/June/July for wildflowers and rhododendron or mid-late October for the fall leaves. Lots of other people like to see the flowers and fall foliage, though, so expect more crowds then. These links might be helpful:
But come whenever you can!
The mountains are great in the summer, but Charleston would be really hot. Outer Banks would be in peak season in the summer and great for beach and water, but the beach is nice any time of year IMO. Charleston is probably at it's peak in March/April/early May. The azaleas would probably be in bloom and the temps would be very nice to warm/almost hot, but nowhere near as hot and humid as in the summer. Ask on the Charleston forum about when they think the best time to visit is. Maybe you could start your trip in Charleston in March and end it in the mts in April.
Edited: 3:55 pm, July 28, 2012