The least of your concerns should be the knives. They are a wear item, inexpensive to sharpen, and not too bad new. I'd be a lot more concerned about the engine. Listen for knocks, do a compression test, look for smoke. These little Kohlers see a lot of strain in a chipper. Check the infeed wheel, should have sharp edges where it bites the wood. Start it up, get the disc up to speed, then disengage clutch and shut of motor. Listen to disc bearings as it free wheels. Should not make any excessive noise. These are pretty reliable but generally unpopular due to their low power and poorly designed single feed wheel that struggles to pull much of anything. If all you use it for is pruning might work ok
