I would suggest instead of doing Sedona for 3 nights, stay in Phoenix or Scottsdale the first night (cheaper). You will be flying a long way and you may be jetlegged. Also, Sedona is a good 2 hours away which doesn't seem like much after that flight may be tiring.
The next morning, on the way to Sedona, stop at Montezuma Castle.
Sedona is expensive so keep looking for a deal The Best Western Inn of Sedona gets excellent reviews on Trip Advisor although I don't know if they have suites. Best bet would to get a roll away cot. Grand Canyon from Sedona is very doable. It's about a two and a half hour drive. There are places just outside the Canyon for decent rates but book ahead as far as possible for the best rates. If you only have one day at the Canyon you will have to stick to the south rim which has beautiful views.
As for a rental car, go on Priceline. We found the best rates (over $500 off every other place I tried).
I noticed on another forum that you have a Southwest credit. Southwest is the best flight to take (in my opinion). They are the only ones (I think) from Buffalo that offer direct flights.
