99.9% of all modders fail to complete anything. There are some exceptional labors of love but they are Labors and Exceptional.So I think the first step is to stop looking at how possible it is by using a game-company development model, and look at the game modding model instead..
So you need money IMO. The trick is limiting costs to that your breakeven is lower. For instance, if a console game doesn't sell 1 million units, it is very unlikely to breakeven. Console game production is more expensive, and time consuming - you have to be approved, there's a whole certification process for the game you have to do as you go along, etc. - bigger audience, but much higher costs.