About the only thing you really need to look at are the rotors/case. If they are scratched toss it. The most common problems are as follows.
1. Plastic coupler inside the snout wears out, causing a clunking sound. New ones go from $15-25 always replace them if you have the blower opened up.
2. Front snout bearing, then second snout bearing. Easy to replace and cheap. You just need a shop press.
3. Rear rotor bearings. Depending on the model you may need to drill and tap the blower housing, add zerk fittings and pump the bearing out with grease. not hard to do. Bearings cost about $3-4 ea.
4. Front rotor bearings. These can be difficult to replace. Sometimes the housing will break when they are pressed out. 95% of the time these bearings are fine and don't need to be touched.