Hi Rahul,
Any bike is a good buy if you like it. Thats all there is too it. It doesn't matter what you buy at least half the people you meet will tell you that particular model isn't any good even though they have never owned or ridden one and probably haven't even seen one before, so don't listen to them.
There is plenty of information about for the Ensign. Try BMS or the VMCC Library. Both are on line.
As for it being in mint condition, don't kid yourself. It might be shiny and pretty but no 50 year old small two stroke is in mint condition. It might be shiny and pretty but unless the owner rides it daily it will have a host of minor little problems, particularly electrical, when you come to ride it.
Most of these problems could be quickly sorted by an rider experienced in British two strokes but are frustrating for someone new to the game.
I would suggest you join the British two Stroke Club who will be very helpful.
Cheers, Bob