Unreal Engine and CGTrader

Don't think I'm reining on your parade or anything, by all means far from it. I myself am using cryengine to build an entire modular city for a portfolio project. But in terms of an actual game, commercial or not, you're going to suffer a lot of quality/artistic setbacks sticking to a free method. Which brings me to the next part, people hopping on board for a project like that. Typically, from what I've seen, startups like that are usually commercial from the start and have a handful of people (small group of 5-7 or so) willing to do the project for a company stake or share. If you're sure you want to start a company, pay those fees and whatnot, register the business, market it, code it, beta test it, and build it, then do it! But I'm just warning you how long of a complex road it really is. Your best starting area would probably be crowdfunding once you get a lead artist to draw up some concepts.

Hope this helps at least a little, again not trying to deter you, but there is a reason that there are decent size companies :)

You need the prototype first and it needs to be good before you're going to get any artist worth their salt and even then it's hard to find someone who will work for free. I've been a part of a number of projects on the side including my own company for a few years and only had one or two successful game launches and by successful, I mean they actually finished. They didn't make any money. It is a TON of work to get a real game finished and even harder to find anyone willing to work with you on a project and stick with you during the whole thing.

The people you want won't want to work on a project in the way you're describing and the people you get probably wont know what they're doing honestly. Free models are great to get your prototype. Get that first. You can even just use boxes for things. No need to be fancy with a prototype. You need to show you can do it first to inspire someone to make things for you.

If you go the crowd funding rout, you will need lots to show and you will need to develop a whole additional plan of attack for reward tiers, etc. I'm not saying it's impossible but it is a very long and difficult road and it's important everything is thought out before you reach each step. Look for things like artists because you have exhausted everything under the sun that you can do and really need them. Same for everything else. Like don't get an LLC or anything like that until you have made a game and are making money to the point where you need to do it to protect you and your income. Here's hoping you find someone willing to work with you and can give you what you need.

Get to know people in the indie community, artists, etc. you'll find connections of like minded people and eventually, someone you want to work with. Good luck to you!