A plow prep package just makes sure the vehicle will support the rigors of plowing.A good plowing vehicle should have the strongest front end possible,HD cooling,trans and PS coolers,a HD trans and transfer case,and a large battery and alternator.If you have a vehicle already,it may have some of these,or you can add your own.
If your looking new,talk to a dealership who deals in trucks,not cars.We spec'd a few truck at the chrysler dealer next to us,who proceded to tell me,i need the trailer towing package,the heavy duty service package,and the snow plow prep package,to go with the diesel,and that i should upgrade to a HD transfer case,bigger 6500 lb leaf springs,and wanted add'l money for every little thing.Turns out,most of the packages overlap,and alot of the stuff came standard with the diesel.See if you can get a copy of the books they have to spec a truck with,and they have detailed breakdown's of what each package contains so you can pick what you need.If they want to sell you a truck,they will give you phototcopies.