I haven't hired outside help, though I can give some food for thought.
Your English here is clear and comprehensible enough, so you could get away with an editor who has solid grammar to just tidy things up here and there. That being said...
...a big part of what makes a comic really engaging, for me at least, is the believability of the dialogue between characters. Does it sound natural? Does it flow well? Does the personality of each character shine through in the way they speak?
If you're pretty confident with your ability to write down a conversation, you likely don't need a writer. If you can read back what you've written out loud, and it reflects your characters well, you're on the right track there. An editor can simply help you tidy up any mistakes.
But if you're not sure you can write a natural sounding conversation, this is where a writer comes in. You can give a writer a rundown of what the conversation needs to include, and the personalities of the characters involved, and a writer can turn that into engaging dialogue.
If you're unsure, have a read of other comics which have good dialogue in them, and read some conversations out loud. Then, read your script out loud. Compare the two. If your script sounds pretty good, and there are just a few clunky bits and spelling/grammar errors, an editor is likely all you need. But if the characters sound unnatural, and the conversation between them doesn't sound much like how people actually speak, you'll want it re-written.