I have worried about this myself, and after spending months researching what is expected once your manuscript gets into the hands of a publisher I panicked!
Thats why I asked my cousin to do the editing, she was an English teacher, and is great at it.
She has fixed just about everything, I actually feel like its her book now not mine!!!!
I dont think I would pay a real editor, their not cheap!
Mind you my spelling is really bad I need all the help I can get.
Also dont fully rely on the editing process of WORD on your computer because it gets it wrong as well!
My advice to anyone who feels they need help in this area is to ask a school teacher, or uni student studying for English, they would welcome the extra money, even if only a little, every bit helps them, but it also give them practice.
Hope this helps.