So I have a private lender now what?

[Mickey Harrison]

I was recently taken back at thanksgiving when 2 family members approached me about my RE Investments.
Them- "so did you buy that other property?"
me- "yep."
Them- " well, you know, if you're ever looking for investors we have some money to invest"

When in the past the conversation went like this.
Them "you know if you ever need help with money(get in a jam) you can always call me and your aunt."

As you can see the difference in tone.

Now to the scenario/question:
I was thinking, for the properties that I want to hold I would agree on an interest rate and maturity of loan with these members.

To do this would you have them do a mortgage 1st position (actually file) if I buy something out right. Or just have a notarized contract and make payments?

or if i find a seller finance deal and my PM funds are used for a down payment.Do I give them the 2nd position mor. or just have a notarized agreement between us?

For the flips I would ideally rather pay them an interest rate. However, 50/50 would be more civil since it's family members and I would have no skin in the game just my knowledge and minimal labor.

What have you all done in the past? Sorry for the ignorance but I've only done conventional borrowing and didn't think anyone would approach me/I would need PM til at least my 5th rental.