I have not ran into this before...advice wanted.

I flipped a condo, which I have done many of times successfully. This one was different in that it is in a bit rougher part of town, but units sell. My agent told me ...

Should I only demand a percentage of total profit (like 10%) or would I simply lend the money to them?

And you will find alot of banks wont lend on properties tore up from the floor up. so you may have to use hard money, fix up and then refinance.

What will make the investors feel comfortable lending you money?

@Adam Stroik talk to JJ Bliss at BD Lending.

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Unfortunately It likely needs hundreds of thousands in rehab which no one is going to want to lend

Local credit union in-house lending program. 10% down with credit union loan (30 year amortized, but 15 year balloon)(5.38%, 30 year) How did you add value to the deal?

Another way to go, maybe try going commercial lending, as mentioned above, but more along the lines of a credit line.

Conventional Loan with local institution.