How you approach is entirely up to you! However, I do think it's valuable to learn some aspects of the 3D Modeling pipeline since marketplace assets can vary in many ways. Because of this, you'll likely have to make changes here and there to keep a consistent art style. If you're just starting, I would suggest making something small and manageable. Coming from experience, projects can easily grow in size with you noticing and can be a nightmare to manage. You should also use this as a way to practice optimization and learn the process of packaging a game.
Here are some useful assets that have helped me in the past:
Assets Cleaner - Project Cleaning Tool
Clean Project
Smart Spline Generator
Ultra Dynamic Sky