When I read the description of this amulet, it said there is a random number assigned to the resistances, and that there may be a suffix for the item.

My question is this: are these factors random, or is there anything I can do to help out my chances at a better amulet?

I'm currently in Act III, and got this stuff from doing many extra runs at the Council Members. I'm a level 29 Paladin, but I am close to level 30. Would I be better off waiting until level 30 (or higher) to make this item? Does the amulet used in the recipe make any difference?

The number for resists is not entirely random; it alternates in the range between 16 and 20.

Whether or not you get a suffix is completely random. Which suffix you get is random, but level-dependent. The higher your level, the more suffixes become available. You won't get suffixes like "Of the Whale" (+81-100 to Life) at level 29.

Arreat Summit website has a section on prefixes and suffixes which lists the levels at which you can make items with these prefixes and suffixes.