Willy, not to throw stones at the Mercury people out there either but Ferm raises some valid fear factors regarding that particular engine. If you figure what that boat and trailer is worth - and I can tell ya I paid $6500 for mine which had a brand new galvinized dual axle and the older Yamaha 150 that I regretibly sold to MJ and turned out to be crap - for 10 g's that would mean the motor is valued somewhere around $4500 - $5000. I don't think so.....
I say pitch up an offer for just the boat and trailer for around $5000 - $5500. If the owner thinks the motor is worth $4500 - $5000, then he should not have any problem getting it by selling the motor separately - right?
Then hang a Zuki 150hp 4 stroke and you will have one hell of a boat!
Best of luck - I feel for your perdicament. I hope like hades you can work it out so we can be V-21 brothers. Let me know if there is anything I can do to help - like take my billy club to the owner's head!
Blue