Let's say I have a number in my country and WhatsApp account associated with it. I want to travel far away, where roaming would be ridiculously expensive. Is it possible that I can buy a SIM card local to that country I choose to travel to, and still be able to use my home country's WhatsApp number?
Yes, you will be able to use Whatsapp with another SIM card and your old account, but do not reset or reinstall the app to not provoke number verification. Source: personal experience; https://faq.whatsapp.com/en/30028016/.