I'd recommend to develop your own style files with a professional TeX/LaTeX developer (which, I am sure, you could find here). Three reasons:
Copyrights. You don't want to risk running into copyright problems by using and modifying existing styles. I understand that you asked for free styles but experience tells me that you end up modifying them and it's often not clear whether that is covered. All these problems don't come up right at the beginning but only when there is some success and others feel they contributed without receiving enough credit.
Marketing. You want to stand out to attract authors. A completely generic layout gives the (false) impression to authors that they could very well just put their paper on their own website.
Customization. You probably have your own ideas about meta data that you want to record and special requirements coming from your field of study for which you need special commands or environments anyway.
Having been in a similar situation I can report that developing our own style files paid huge dividends. Our documents look better, LaTeX code is much more robust, and we receive praise from authors. For anyone curious, you can find our Open Access publications in computer science here: Leibniz International Proceedings in Computer Science (LIPIcs).
Addendum: to be fair, it took us a while before we reached this point where we developed our own style, obtained DOIs, obtained ISBNs, organized long-term archival, set up an editorial board, and so on. It was done by a colleague who had worked for a scientific publisher and had clear ideas about this. I think it's perfectly fine to start small but if you are serious, you will have to think about these aspects at some point.