Stormer,
I tend to be very conservative when it comes to partnerships. Not just in aircraft, but business or anything else. My $0.02, is
always have things in writing. I would do this even if I was partnering with family or very close friends. Everyone needs to understand the expectations of the partnerhsip and how to terminate it. Don't ruin a good relationship, buy "winging" it.
I'm sure there are some here who have sample partnership agreements that they have used in the past. AOPA has some info
here as well.
Good luck!