Am i right that there are many C trains from Huaihua to Changsha?

Well, I'm afraid not. Instead of C trains, there are many D trains (bullet trains) from South Railway Station and get off at Changsha South Railway Station. And the trip usually takes around no more than 2 hours.
By the way, the ticket fare of second class seat is CNY152.5 and first class seat is CNY256.5. And the schedules are from 08:30 to 21:41.

Dear Iris,

Thanks for your reply. Are tickets easily available on the date of travel? I will be there on 9 November 2016. Or do I have to buy the tickets in advance? If it is the latter, can I purchase the D/G train tickets in advance at another train station i.e Zhangjiajie which does not have D/G train services.

Yes, it's ok for you to buy the tickets on the date of travel even if at other train station.
You are suggested to book them in advance. You can ask help for TravelChinaGuide because the official website doesn't have English version. If interested, their contact information is bookingtravelchinaguide.com.

I would rather not book advance thro' a booking agent as my plans are not firmed and I do not wish to blow away 4 tickets if we do not take the train. I will be in Zhangjiajie for 4 nights before going to Fenghuang for 2 nights and then go to Changsha. When I am in Zhangjiajie I would by then have firmed up my travel plan from Fenghuang to Changsha (it is either by bus or train).
Would there still be available tickets if I were to purchase it 4 days in advance from Zhangjiajie train station?
Any idea the time for the bus journey from Fenghuang to HH?

1. First of all, please note that there is no C train or D train to Changsha from HH. But there are many G trains. I am sorry for the mistake.

2. There may be some tickets left. But the schedules may not satisfy you. You are advised to book the tickets in advance.

3. You can take a bus to HH West Bus Station from Fenghuang Bus Station. The duration is about 1.5 hours. Upon arrival, take a taxi to South Railway Station for about CNY 40.