you dont have spend a fortune, if you go to your nearest gunsmiths, they normally stock a good selection of second hand air rifles. in regards to pellet size, theres no need for .25, .177 or .22 is down to personal choice, .177 has a flatter tradjectory and higher velocity. and the .22 has more of a punch. i prefer .177. you can get a good springer for not much. the best way ive found for slapping some rats is sitting about 20 yards from a run, with some bait set on it. though dont use any solids, use somhing like peanut butter, or blitzed up cat food{ your mrs might not like it, but the rats do}. liquid baits give time to take a shot as they cant scurry off with it. use some cover so they dont see you moving, or if your gonna shoot them often, its a good idea to get a sight mounted lamp or lazer sight.