This is really a two-part question: established writer vs. novice writer, and publishing a novel (self or traditional) vs. publishing short stories (lit mags/competitions).
You don’t have to be an established writer (established how, in what sense?) to publish a novel. Anyone can self-publish and anyone can query an agent or publisher. Are your chances greater of getting accepted for publication in a traditional setting if you have previous publication credits? Probably. Maybe. It really depends on what those credits are. What seems to be of more importance for fiction writers these days is a focused and curated social media presence.
As for the hiring of an editor, if you are an established author with a publishing contract, the publisher may provide an editor for some level of editing or may provide an advance for the purpose of hiring an editor (they may have recommendations). However, even that is becoming rare these days and the quality of final product (your book) is completely in your hands. That doesn’t mean you have a chance of getting a poorly edited book accepted by a major publisher or agent. What that REALLY means is that agents and publishers are looking for POLISHED manuscripts. So that really puts even the “established” writer in the same boat as the novice.
The question of whether or not you need an editor is a simple one. If you plan to query a novel with an agent or publisher, you need an editor whether you are a pro or a novice. If you plan to self-publish, you need an editor. If you plan to submit work to a contest or literary magazine, you need an editor. Every publisher wants a polished manuscript. Readers certainly do—errors distract them from the stories they crave.