By professional editor, do you mean an editor at a publishing house or an editor I pay to work on my book? If it's latter, what does this have to do with how you view it with regard to offering presentation? Not being catty, just not connecting.
Of course the best solution is to have an in-house editor work on a book during its publishing process. Beside the fact that these editors know exactly what they are editing toward, there is added pressure on the author to get it right or else. However, if no contract is offered and a writer’s work is rejected time and again, the only solution may be to find out why. There are a number of courses of action available to writers in this case and one might be to hire a so-called book doctor.
If this is deemed to be the solution, a writer should first consider an editor who is directly connected to the publishing industry. In my opinion, to make a book ready for today’s markets it must be edited for both content and commercial appeal, the later of which would be difficult for someone who is not directly connected to publishing.
To answer the last part of your question, yes, a writer will have to pay to have her book edited by any professional editor or book doctor.
With regard to offering representation, I put all the weight writing quality no matter if a book has been professionally edited or not. If the writing is great, I next consider if editors I work with are looking for that type of book for their lists. Only if the book meets both criteria, will I offer representation. The reason I don’t weigh the professional editor aspect that much is because no matter who offers editorial comment on a writer’s work, it’s still up to the writer to make the necessary changes. After all, an editor does not author the book; she only attempts to persuade the writer to fix problems within it. Also, a writer should be able, with the help of a good editor, to go beyond the offered suggestions and make the book even better after learning where its major problems lie.