Mandarin in Hunan Province
According to some sources citing the Ministry of Education, diffusion of Putonghua （普通话）in the country has reached 70% in 2015. 

The biggest issue that you might face in Hunan (and in almost any other province) is not that people would talk to you in dialect, but that they would talk to you in Mandarin with accent. 

Hunanese accent has a few distinctive traits, some of which in common with other southern speaks: 


  zh/ch/sh become z/c/s: 诚 sounds like 层 
  eng/ong --> en/on： 朋 sounds like 盆 
  n --> l (el)：里 sounds like 你


and as others have pointed out:


  h --> f: 湖 sounds like 服


That's not a small hurdle for new learners. How quick you'll get used to it depends mainly on your proficiency level. With an HSK3 you might have some hard time at first. Especially the n --> l change is tough for Mandarin students.

One last note: asking for references to learning resources is somewhat discouraged here as it is difficult to maintain lists and links. I believe that you can find something by querying for 湘语 on Baidu. Good luck!