There are only two people I woudl co-own a piece of equipment with. Reasons there are two people is you have to consider the other owner has as much right to it as you do, with that thought alone you want the other owner to operate equipment safely, properly, not severely overload it, help maintain it (by that I mean if you grease everything properly, check fluids, check tires etc that if you did it or the other wonder did it you'd trust it to be as good or better work than yourself), decide to not "loan it to people" without one of the owners present (ie: if my uncle wants to borrow it that's ok but I (or the other woner) come with it).
Buying costs, are you financing or buying cash? How financially stable is the other person?
Maintenance and repairs: Who is the mechanic of the twosome, not who thinks they are but who is? Determine who is the smarter of the two and you've got a great start already! How are costs divied up? Who's resposible for what when it breaks when you use it or he uses it?
Now earlier I said there are two people who I'd co-own equipment with, I shoudl have said three, anyway one is my dad the other is a good friend who farms about 2000 acres and the third is my uncle who lives on the other side of the farm. My dad and I are in a little bit of everything together. My buddy the farmer also owns a hunting club I guide at and I work on his farm often during harvest and planting season and off an on with various things.
Currently my buddy the farmer and I are looking at buying a Bobcat together, we use the same banker and when I have talked to the banker he has said just tell me how much. Anyway between us we've got the knowledge for repairs and maintenance (I'd say he has the upper hand in this department due to his 40 years of farming vs. my 26 years of living). As far as costs we've decided we'll split 50/50 on repairs, when we use the machine ourselves we are probably going to say every machine hour you owe $10 to the fund, obvioulsy if we hire out with it (like snow removal) we'll have devised a way to work that out (currently we haven't).
Basically I am saying I will only co-own something with someone who I know is going to treat it and takecare of things in the same manner that I will.