Yes definitely u can start your business from Varanasi, It's a developing city , everyone wants to invest in Varanasi. You can become a builder as u have the land (20 biswa is more than enough to start) or you can set a solar plant. 

to become a builder u'll easily get investors as well as the local support bcz now a days property prices are likely to hike ( 2BHK will cost Approx 60-70 lacs)