My first handgun was a 4" GP 100, just like yours. I cannot remember what I paid for it, but what you paid sounds more than fair for the value of the gun. I have always liked Ruger firearms, and the GP 100 is a great representation of their quality. I had a trigger job done on mine, as I always thought that it was a little heavy. I don't know what type of white tail hunting you do, but the .357, especially out of a 4" barrel is marginal at best. Now, I know that there are many that will disagree, but my personal minimum is 41 mag. That said, if you pick your shots at close whitetails, any heavy loaded softpoint or hard-cast lead bullet should do the job. You may try ammo from Cor-Bon. Hornady just loaded their Evolution bullet in .357 mag, and that looks like a winner as well. Nice gun, and good luck!
K.K