You need 2mm nipples for Alpine III, as they are 14g at the threaded end. 1.8mm nipples are for 15g spokes. Prolock nipples are like nylock nuts, they don't loosen when unloaded. If you're doing the kind of big hits which make some of the spokes go completely slack, they might be worth it if you don't want to be truing your wheels every week, but if your hitting that hard you'll be truing them every month anyway. Very small gain for your extra £19 when the nipples which come with the spokes are fine for 99% of the time, although as you're already going for the best of everything you might as well go all out and get Prolocks so you'll always know you didn't make any compromises anywhere.
You need 2mm nipples for Alpine III, as they are 14g at the threaded end. 1.8mm nipples are for 15g spokes. Prolock nipples are like nylock nuts, they don't loosen when unloaded. If you're doing the kind of big hits which make some of the spokes go completely slack, they might be worth it if you don't want to be truing your wheels every week, but if your hitting that hard you'll be truing them every month anyway. Very small gain for your extra £19 when the nipples which come with the spokes are fine for 99% of the time, although as you're already going for the best of everything you might as well go all out and get Prolocks so you'll always know you didn't make any compromises anywhere.