The PA28-140, 150, 160, 180 are all fixed gear, and the three digit number is the HP of the airplane. If there is a '1' as the third digit (PA28-161) then the airplane has a partially tapered wing. A PA28-160 would be the same basic airplane, but with the older rectangular 'Hershey Bar' wing. There are other differences as well. Some of the lower HP ones, like the PA28-161 Cadet model did not have a baggage door to save money, as it was for the training market. There were also different body lengths made. In earlier years the PA28-180 was shorter, then back in the late 60's or early 70's they lengthened the fuselage to give more leg room to the back seaters.
As far as the Arrow goes, these are the PA28R's. The same goes here as well. A PA28R-201 has 200HP and a partially tapered wing. Some of the Arrows had turbo's, others had T-tails. The only one I have flown is the Arrow III and it was a decent airplane. They are kind of slow, but you can carry 4 people and 50 gallons of gas.
You should probably be able to find some info on these airplanes on the web. Trade A Plane would be good, as you can see what the differences are in the pictures and descriptions of airplanes for sale.