I'm sure glenjanie and Square Eye are trying to be helpful, but taking recommendations from people on the internet about the structural portion of your house could be a poor decision. Hire a structural engineer, they'll figure everything out for you for likely < $600, cheap peace of mind if you...
Sounds like you need a structural engineer. Let me see if I understand whats going on. The second floor is supported by 2x10 @24" o.c. floor joists. The roof consists of 2x4 @?" o.c. rafters with 2x? rafter ties. If the rafter ties define the second floor ceiling plane, then you are talking...
If I were the buyer I would not accept structurally dependent repairs done buy the owner. Sill replacement runs about a $100 a foot in my area. Hiring someone will let the buyers know its done professionally and safely.