Well I did the same when I got into my partnership. There will be 2 that are going to pay for issues, but still, you don't want to fall on someone else's problems, one who is looking to share his nightmare if there is seriously something wrong with the plane. That would make for a very bad start of a partnership.
I woulld say at least to the following :
1 : Make sure aircraft is not out of annual.
2 : Have a visual inspection, if you don't like what you see, walk away, if you do, go to #3.
3 : Review all the log files, and again, walk away if there are serious stuff.
4 : Do a PPI with the mechanic you have choosen, tell him to give you all the reasons why you should'nt get into this venture, don't let you or him fall in love with the aircraft.
5 : Make sure you can talk and get a good feel from your future partner, ask around the airport what kind of person and pilot he is.
6 : Get a purchase agreement that is reviewed by a lawyer, the couple hundered dollars of costs can save you plenty later.
7 : Make sure to have a signed partner agreement, for engine overhaul costs, repairs, what is permitted or not to both partner, how insurance will be paid and all...
Good luck and enjoy...