Choose hardware (card and wifi)

Hello,
I would like to have a Piratebox. I have a good skill on Linux systems, then I think Raspberry would be a good choice for me to adjust it to my needs, install services... But this platform is not primarily intended to be a wifi router, then I should use an external wifi card. What USB card (or chipset) do you advise me to use?

I already own a Pi 3B+ but I need a new one. Should I prefer another model? Like the Zero W? I don't know if this card will really use less power, it can be great if I use a powerbank.

The pi is a good choice, yes. It's readily available, low cost, and will be around for the foreseeable future.

You do not need an external wifi card, though you may want to use one anyway. The 3B and Zero W both have wireless capability built in which is quite capable of serving as an access point. You can use your 3B+ for testing purposes, but if you intend portable use then you would be better off buying the Zero W as it is both physically smaller and consumes far less power. I have built a piratebox using a Zero W myself, and can confirm that even its limited processor performance and memory and quite sufficient for the task. You'll get maybe three times the battery life with a Zero W compared to the 3B+.

There is one drawback to using the onboard wireless, though. Due to the tight space constraints they use a very small antenna, and achieving such size does compromise performance - which translates into having a reduced range. If you intend for your piratebox to serve over a large physical area then you may want to either modify your Pi for an external antenna (This requires carrying out some incredibly precise surface-mount soldering), or use an external USB adapter. You may want to evaluate the performance of the onboard wireless first though, and decide if it is sufficient for your intended use.