Two customers want in on my first flip. How do I say yes safely?

The line that jumped out is that you don't need the money. That's the strongest position you'll ever negotiate from and it argues for the simplest structure, which is a loan with a written note, a fixed rate, a stated term, and security against the property.

Equity means they share the upside and the downside, and you'd have to explain to a kitchen customer why their 40k came back as 31k. A note means they get their money and their interest and the outcome of the renovation is your problem. If the number that keeps you up is what happens at a 270k sale, a note answers it cleanly.

What a note looks like in practice and whether it needs to be recorded depends on your state, so have a real estate attorney in your state draft it. That's a few hundred dollars against 80k of other people's money.

Something I'd add on the 58k scope. You said you're doing most of the work yourself. Price the version where you get busy with paying customers in month three and have to sub out the whole finish package, because that's a real number and it isn't in the 58k.

If that version puts you at 78k of renovation and the house sells at 290, your margin is thin enough that 5,300 of interest matters. Run it before Thursday so when he asks what the money's for you can say what it protects against.