at black city area and route 18
The starter Pokemon are never encountered as wild Pokemon.
If you transfer one yes. Pikachu are NOT wild Pokemon in Black and White.
Trade it with people who have Pokemon White version as you cannot find wild Rufflet in Black
it makes wild Pokemon less likely to appear
Braviary (as well as it's pre-evolution, rufflet) does not appear in the wild in Pokemon Black version. You will have to trade with somebody who has Pokemon White version. Likewise, Vullaby and Mandibuzz can only be caught on Pokemon Black.
One of the only differences of the games is that they both have an exclusive area. Pokemon Black has Black City, which is full of trainers to battle. Pokemon White has White Forest, which has both trainers and wild Pokemon that are exclusive to White. In other words, White version has more kinds of Pokemon, so I'd say it's your best choice.
Cyndaquil cannot be found as a wild one or a gift in Pokémon Black.
You can't find thundurus in the wild in black. You need to trade one over from white version or GTS.
You can only get him if you have caught him in Soul-Silver or Heart-Gold. If you have done so you have to trade him into your version of either Pokemon black or white. He is not obtainable through the wild.
You must defeat wild pokemon or pokemon owned by other trainers another way to level them up is by using the item rare candy.
You cannot catch Tepig in the wild. You get it from Professor Juniper at the beginning of the game or have someone trade it to you.
Charmander is a fire type of Pokemon. It is not possible to catch one in the wild in Pokemon Diamond version.