Scrivener is a very competent, though initially rather confusing, software that enables you to write a longer text in a more organic fashion. By providing a binder of content, you can write your text in small snippets in any order, organize it freely and later glue it together the way you prefer.
While roaming charges tend to be high (€2 or $3 per MB), there are prepaid SIM cards in most countries. One of these will usually set you back a mere €10 and offers from 1 GB and up of data transfer. This is usually enough for at least a few days of use. But instead of putting it in your phone and using its hotspot feature, and not being able to keep your home phone, buy a portable WiFi router. These come with built-in battery which lasts up to five hours and provide a local WLAN network which up to five devices can connect to.