Hi, welcome! You see, in black version the only pokemon you can find in the wild are the newest, generation 5 pokemon, UNTIL you have beaten the game (ie the elite four and champion). Then in the new areas you will be able to find some older pokemon, as well as outbreaks in any areas. As for wifi, if you are talking about the standard GTS (not negotiations), then yes, it only lets you search for pokemon you have registered on your pokedex, either by catching or just seeing in a battle. However, if you go to GTS negotiations you might be able to find someone offering an older pokemon or one you haven't seen yet. Or, you could try to find such a trade partner in this forum under the gen 5 trading section! And one last thing, don't call the pokemon "monsters"...yes technically they are "pocket monsters", but MOST people who are fans of the series, at least in America, don't use that term. Just saying :-).
I have 2 Friend codes I use about equally. One is 4055-3723-1345 and the other is 0576 3520 6954. I'll try to mention which one to look for when needed, if I don't then ask.
I try to train pokemon with the best natures and IVs, but generally don't breed for IVs and occasionally use a pokemon that could have a better nature out of laziness (on the other hand I have reset or recaught the same species 20+ times to get the best nature and decent IVs sometimes). I somewhat do EV training, but don't calculate it out so it's perfect, just rely on the chart in the super training thing.