If you just bought the house in a marketed / arms length transaction for significantly less than the assessment that’s good backup in itself. The town should have record of the sale but bring some evidence of the sale for the file. Appraisals are also extremely useful - definitely bring a copy of the appraisal. You could look at comps but I would focus heavily on the recent sale and appraisal. They will go much farther in establishing the value of your property than what your neighbors house may or may not be worth.