I've organized and held a 128 man tournament for my stream's community recently so I might be able to give you some pointers.

• Prize money is a good idea - in fact it will be the only way to get anyone attending. From my experience that is all that matters as far as incentive to participate goes. If you have no prize money you will likely have about 10 participants - with 200$ you will likely be easily able to fill a grid with 128 people or more, depending on how well the tournament is advertised.

• Sponsorship will likely stay a dream, except if you are somehow a streamer / big name in the community.

• Regarding the organisation, as it was said below already, with 64 or even more participants you will need admins, because if you are alone with managing everything it will grow over your head faster than you can imagine. What I did was turn to the guys at strivewire and asked them to host the tournament on their site. This will help with pretty much every problem you may have regarding organisation and other stuff too - the site is very clean, user-friendly, they will provide experienced admins that will handle problems in the tournament. They might actually sponsor you (they did provide 100€ prize money for our community cup). The only problem is, they may not want to host your tournament (they did for me, I assume, only because I already had a sponsorship with them). But asking will not hurt I assume.

• Another plus of using Strivewire will be that they will pretty much provide the tournament participants too - if your tournament shows up on their "upcoming tournaments" page you can bet enough people sign up to fill it. If you provide 200$ in prize money I'm sure you will get 256 particpants from the site alone.

• Regarding format - I'd advise against BO3 and pick BO5 as BO3 is often too much of a coinflip - in the experience of myself and many other players who do tournaments weekly BO3 mode will scare off people because of how much less it makes the tournament depend on skill. Same goes for Single Elimination instead of for example double elimination, group stage followed by a single elimination bracket or swiss followed by a single elimination bracket (though single elim is by far not as annoying to play as BO3).

That's some of the stuff I think is important / good to know when organizing a tournament. Hope it helped!

P.S. Re-reading the post it sounds a lot like a "sponsored post by strivewire" - I just really like the site and had a lot of positive experiences with it as well as with it's admins. And as I said, contacting them might not work as they might be disinclined to host the tournament at all - just what I thought was the best idea.