I would recommend the N5.0 highly, it is a fun boat, faster than P16 upwind, but about the same speed overall.
As for what $$ to offer him for his boat, consider some other alternatives.
- Nacra 5.0 has almost no parts in common with P16. Now you have two half boats, both of which need parts. Why not find a P16 with a good mast and bad hulls.
- why not source a used P16 mast. have you tried sailingproshop.com?
http://www.sailingproshop.com/prindle16parts.htm
http://www.sailingproshop.com/usedcatamaranmasts.htm
- Repair the broken mast - there have been several posts on how to do that, and if you can't find them, post again, you'll get lots of ideas/help.
Good luck,
Jim Casto
NACRA5.5 uni/sloop