Hi Jane,
I think most would be okay to read a fully-written draft, with complete references and abstract, just give them plenty of time and of course don't assume they'll be too eager. That way, they can give you feedback in a similar way to the official peer-review editorial process. They are also very willing to point you in the direction of a journal that would be the best fit, as they have much more experience navigating the journal aims and submission process. Some journals are more accepting of early career researchers then others, as it gets to 'double-blind' process only after the initial editor's glance of seeing if it is worthy to pass to peer-review.
Anything more than that may step into co-authorship, which includes developing the argument or reading over incomplete sections and writing on them/suggesting the direction. Of course, you can ask them if they'd be willing to co-author.