Well, the hard part is finding people who can code that are willing to work on a project like this without making clear what kind of compensation you are offering up for their time. You'll need to decide if you're gonna pay per hour, per line of code, per completed milestone, or "profit share" with a percent of whatever you make at the end of it. It will be far harder to get someone on board who is willing to help for free, even as a passion project.

As far as the engine, what you'll need to look into is which ones have various free/cheap resources you can make use of to cut down on the time investment involved. Unreal and Unity both have pretty large catalogs of premade resources, some free and some for sale, but have major parts of the license agreements you need to read in full before trying to make use of them - especially if you expect it to break a certain point in total sales. Godot might be an alternative engine to consider as well, it has fewer resources available, but probably the most permissive license possible for a game engine - the M.I.T. license (use is free, you can make whatever money you want off of it without needing to pay them anything). The downside of Godot is that you'll have a smaller community to reach out to for help with specific coding issues, primarily through reddit.