Land/advice wanted in Essex/kent/southeast

greetings everybody
complete newbie here please help and bear with me,
I'm looking to purchase a small piece of land on a farm rural land in the south east , i plan to build a pole barn type structure or bungalow for me and my fiancé to live, i have no idea if this is at all possible or legal etc and how much it would cost etc, can anybody please give me some advice id really appreciate it, i don't know where or how to begin , many thanks