Correction to my previous post - Purism will not do everything for you. You will have to set up WhatsApp yourself. I've looked at the Librem.one website. You get a secure and publicity free email account if you pay. That's it really. To be fair, they also set up a VPN tunnel for you. It's actually just a repackaged OpenVPN, but at least Purism set it all up for you.
Personally, I'm pretty useless at technical stuff. I still haven't managed to set up 'bridging' to WhatsApp etc. from my Android device. Despite this I'm going to give Pine64 hardware a go. I do so under the strict understanding that I can forget about bothering customer support if I have any problems, because there is no customer support. It's dirt cheap, so I'm OK with that.

Purism's Librem 5 and Facebook's WhatsApp – a curious combination! Yes, that's possible. There is more than one way to do this:

1. The most pain free way – leave it all up to Purism. They will take care of everything. At least, I presume they do this too. You have to pay for it though. I think it's about 8 euro per month for their full software support package. Now, normally, the kind of software they offer is already totally free anyway, but if your time is more precious than your money, then this will make life easy for you. Actually, this is quite good value. For example, if you choose the paid version of a good email service such as Tutanota, it's €12 per month. Purism provides a full package. Matrix is part of that package, and that can communicate with WhatsApp by 'bridging' (look at the Matrix website).

2. The normal way – install WhatsApp onto your device (only available on Google/Open Android, Apple iOS, Apple Mac and Microsoft Windows. Either wait for Facebook to make it available to your operating system of choice, or install Android onto this device. Then open an account on WhatsApp if you don't have one already in order to communicate with other WhatsApp users.

Can't wait for it.  The only question is whether it will have whatsapp