Seller financed is one of the oldest tricks in the book. It means you (the seller) pay yourself for the sale from the proceeds of any furture profits. There are a couple of other red flags in what you wrote above, like time pressure (always a bad sign) and "the ratchet". Of course you may be informed and able to speculate, but there's nothing better than getting yourself a very good legal firm. Good ones are very honest about the downsides to any situation, which is what you need. Nothing is worse than being told everything is fine only to find out later that there were foreseeable issues.

You should go into this with a take-it-or-leave-it approach and call the shots so that it's what you want, and not what they want to give you. But you'll have to get a standard position from everyone of your partners, and the only way to do that is to say NO to dealing with individuals. The reason is divide and conquer. That's another thing they'll do.

Also DO YOUR RESEARCH. Who are the buyers? Can they give you personal guarantees to pay you even if what they're doing does not work out? Who is this mysterious competitor? Why didn't you know about them when you started your project? Again good lawyers will help you out here.

So form a company, set your equity, get the best legal advice there is, and do not have multiple positions on your side when you go into negotiations. Be prepared to walk when you don't get what you want.