there will be enough memory for the above tasks?
The Macbook Air will suffice you with a head for such tasks. In fact the memory will be available to you in the order somewhere 90Gb, can dozens of movies to download without problems, it is unlikely you will be to download 4K (because it makes no sense), and FHD good quality 4-8Gb is usually, although such a screen 720p is enough IMHO!
will the Mac all also work well after 3-4 years?
Yes, no problem. Still, demand for MacBook 10 years ago. They work fine so far...
is it worth to pay for a year/two guarantees?
Just reading the reviews in advance about the store. This guarantee from the shop, and how they perform their warranty conditions - you need to only know in the reviews! Apparently you do not want to buy the official MacBook (sold to Moyo, Eldorado, citrus, etc.), so the Apple warranty will not act in Ukraine in the official service , in the event of a breakdown the nearest official service which will take to repair your MacBook in under warranty for free Poland or better in Germany. Well, there are services which, for a fee (150$) carry your MacBook in Europe for repair and back. On Macbook Air 2019 is valid extended warranty on the keyboard but it is only in the Apple services, your store can simply refuse to repair at the expiration of the term granatino from them (3 months). Because compare price with extended warranty with official price in the Moyo/Citrus, now there promotions and most likely more profitable to take the official.