Bigger than 17" with that straight top tube. I reckon 19". Not my favorite shifters / brake levers for that era, but that Exage driveline is nice enough.
Original tires? My 'Hopper had Specialized tires that looked just like that.
Prices for such are all over, over here. I've seen them as little as $25, or even free, and as much as $200. Personally I'd hold out for $35-40 as mentioned, but that much bike, that clean, for $50 is nothing to sneeze at.