Going with a friend to look at a 525SC with 50 hours after a rebuild. Besides the obvious compression/leakddown what else should I look for? Especially the blower? I'm not familiar with blowers.

The blower lube reservoirs will get gas contaminated as the seals wear. Lots of people dont change the lube.

Drop the blower belt. Grab the pulley and give it a spin, listen for grinding sounds. Feel the bearing as you turn it slowly, is the action smooth? Shake the pulley back and forth to check for slop in the drive gears, listen for clanking. Pull out the little screwed in dipstick on the front cover and check the lube for bits and pieces and gas smell.