Laptop choice and Do I need an external monitor?

Hi

I have a couple of questions that I need help with please.

Would a 17" laptop with a 72% gamut screen be good enough for photo editing, or is it still beneficial to get larger external monitor?

If so, what are the reasons for an external monitor?

Alternatively I can buy a 15" laptop with a 90% gamut screen, I am undecided whether to buy this or the 17". Both with the possibility of the Dell U2412.

What setup would you suggest please?

Please note, that I cannot use a desktop instead.

Many thanks.