The max amount of EVs you can have in a stat is 255, and the max you can have overall is 510. (In B2/W2, calling Bianca will tell you if anyone in your party's hit either of those limits.) As a result, you can only 100% maximize two stats and put minimal work into a third. You can heavily invest in all three of HP, Def, and SpD, but not maximally, and make sure they're all multiples of four (EVs work in rounded-down groups of 4, so having remainders in a stat is pointless; because of this don't bother adding more then 252 EVs to a stat).
It does not matter when exactly you use your EV-increasing items, as stats in Gen V are recalculated after every battle. That said, with a low-level Pokémon, you will have a hard time noticing any stat changes as you add EVs: at level 100, every 4 EV points adds 1 to its stat, and things scale uniformly for lower levels (so for example at level 50 you'll see a stat change every 8 EVs).
The Macho Brace (and related items), as far as I am aware, only affects EVs you gain in battle, and not those you gain through EV-increasing items.