I would not use a laptop for mining at all.
I have seen external GPU on a laptop used for gaming but mining for months constantly I can't see being a good idea and don't forget you would need a external power supply also.
You power cost is fairly high during the day also so maybe you could mine during the night as your nighttime cost is low I think. I am in UK and electric is very expensive here but I have the cheapest possible I believe 0.11GBP.
My opinion is if you want to start mining and you're already going to buy a GPU and Power supply then you might as well get a motherboard, RAM, CPU and a data-stick also. Wouldn't be more than 100EUR more and leaves you with your laptop to use.
The best GPU for efficiency would be probably AMD RX 470/RX 480
For hash rate probably AMD FURY, NANO, PRO DUO
Nvidia 1070/1060 I understand is extremely efficient and gives a excellent hash rate for ZEC/ZCL