If you're really sewing regularly and want your projects to look more professional, an overlock is definitely worth it - but not because you need one, rather because it takes work off your hands and the finish is noticeably better. The Instagram pictures are just heavily filtered; a regular sewing machine with proper seams works fine too, but finished edges just look more polished and last longer.
For beginner models, I'd go for 3/4-thread overlocks - they're simpler to handle than 5-thread versions. You adjust the thread tension and stitch length, and then you're good to go. You don't need expensive professional machines; cheaper models do solid work too. What really matters is that you're actually sewing frequently beforehand, otherwise the machine just sits around and you'll regret spending the money.