It's very individual. The big bugbear is if one is going to need data whlle going to a punt (e.g. for maps).
Three do a plan with unlimited data but of course it's not cheap and leaves a paper trail if your o/h is reading your bills.
"Burner phone" on the other hand can imply more extreme than needed.
This is the standard 'Snowden' rule:
What do you want to protect?
From whom?
What would be the consequences if those person(s) found out?
Whatapp is owned by Facebook.
If you see their profile picture means they have saved your number in their contact list and if you don't see any pic that means they haven't saved your number. UNLESS they don't have a contact pic (which you shouldn't either of course). Better still, don't enable it to see your contact list at all. Whatsapp also syncs to certin other apps automatically 'for you'. That includes Instagram, Google Plus and a host of barge-pole apps. And remember to switch off Whatapp cloud backups.
As far as I know, blocking all messages from unknown numbers is not possible on WhatsApp. But if you receive a message from an unknown number and want no more messages from that number, you can open that particular chat window and block the contact from there.
Jeff Bezos’s phone was compromised by a malicious video sent via WhatsApp, which is probably not a common concern unless you have upset a cyberwarrior p4p (there are a few and they are dangerous). I trust seasoned punters, especially travelling abroad, know not to fall for whatsapp scams (less frequently pictures I think, more like links ti websites or 'upgrades' that then mean you get charged premium rates for incoming calls.
A small but relevant thing about using Google maps is that it will try to save everywhere you've been unless you disable that feature (in which case it probably saves it without telling you but at least you've reduced). The more things you search for, the more Google will suggest such things to you when you start searching – or when somebody else starts searching on your browser.
Some countries have the right to search your phone at customs. Some of those same countries consider seeing a prostitute an illegal activity. DuckDuckGo is your friend!
Oh, and remember to turn off previews in your SMS messages.