Colin,

Here goes:

Campden tablets = sodium or potassium metabisulfite and are used for killing off wild yeasts in raw grape juice (must). Alternately, you could heat the must to pasteurization temperatures or start with concentrates / pasteurized juice. You can also use yeasts with a very high competitive factor (like the Lalvin EC-1118 or K1-V1116) and hope that they'll overrun any wild yeasts but you're taking your chances there.

Yeast Nutrients = anything from specially prepared dead yeast to combinations of diammonium phosphate, dipotassium phosphate and magnesium sulfate. This is used to make sure the yeast has enough to eat, especially nitrogen. If you're doing grape or fruit based wines, it's not needed though it might speed the fermentation a little. If you're doing things like mead, you can substitute a handful of raisins.

Acid blend = a blend of citric, malic and tartaric acids that usually comes in crystalline form. It's used to balance the acid in a wine as determined by pH (usually tested by little strips of paper that turn color or a rather exepnsive electronic tester). The acidity affects the taste so that low acid levels taste flat / flabby and high acid tastes sharp. If your pH is too high, you can add the juice of a lemon to the fermentation (or later when you first taste the wine) and be just fine. If you've got a good balance of grapes, the fermentation should be fine without it.

Good luck on your wine and don't worry too much about the additives ... people have been making good wines without chemical additives for a long time.