1 - slug pellets are ok and wont infect the spuds if placed on the surface. You can get less harmful ones now which are fairly safe. A copper band might also work.
2 - Normally yes unless you get an early variety which can be ready as they come into flower. (probably too late for those now anyway)
3 - you dont have to buy seed potatoes but it is recommended as they are certified clean and free from disease and are more likely to give you a better crop.