I can suggest you this guide about freelance marketplace https://azbigmedia.com/business/building-a-freelance-marketplace-a-comprehensive-guide/. It's profitable niche today. First of all you need:
Do comprehensive research and find your niche
Define a demanded set of features
Think about the app structure and interface
Database development
Custom development is the best option in any case. i think when it comes to software building it's better to hire pro tean to save time and money and build a high quality product.I can suggest you this guide about freelance marketplace.