Once a book is written, there are many tasks and I like to either do them or manage them all. This is how I make certain my book will remain mine. If you can do everything yourself, you have complete control but not everyone can do that. You can still control the final product as long as you are the publisher. Those aspects that you cannot do can be contracted to someone else but you still make certain they deliver what you want.
Contracting for editing is a good idea but make certain you have edited your manuscript as tightly as possible before having someone else take over the editing. The more you leave for the Editor the more it will cost you. I suggest you pay someone to do a chapter or section to make certain they understand what you are looking for.
The other area where assistance is often needed is with graphics, especially covers. Unless you have good creative graphic skills, you should be looking for an artist for your cover.
Do as much as you can and then contract the rest to honest, reliable, and recommended artists.