Hey there...
That price doesnt sound too shabby - I believe my 99.5 is worth about 9-10,000
You can always check autotrader.ca to see what similar trucks are going for.
AT tranny - we're not known to have the same issues as the older pathys. My only issue with the AT, is the lag accelerating on the highway. A manual tranny would fix that right up, but they are somewhat rare.
As mentioned, we dont have major rust issues... only ones Ive heard of are the window seals (esp the rear), and the step rails. Mine has some on the pass side front wheel well.
I've changed all 4 of my O2 sensors, I dont know about them failing early, but they did fail... but not a huge issue to deal with.
Good luck!