Welcome to the forum.
The 05-06 900 was a serious problem child for Polaris. When that engine came out, it vibrated big time and some riders went through a belt a ride. There were many recalls with lots of dead sleds heading often to the dealers. There were bad motor mounts and fuel mapping issues galore, especially in the 05 version. The engine (actually 866cc) had plenty of torque but the Cat and Skidoo big blocks were still more powerful and held together much better. Polaris came out with the small version of the engine (700 but really 755 cc) in 2006 and dumped the entire engine by 2007.
That's the bad news. The good news, with the horrible rep the engine has, you can pick them up fairly inexpensively and you may get lucky and get a good one. When they run, they run very well in the hills.