For Route 5 the lowest level is 19.
No, it is impossible without the Action Replay.
Level 20.
Yes, the lowest level Pokemon can be is level 1. There are, however, some Pokemon, such as legendary Pokemon, that are only found at a certain level, and cannot breed.
Level 64
Level 43
Level 5
futachimaru evolves at level 99 in Pokemon black and wight
Klink evolves into Klang at level 38 on Pokemon Black.
krookodile evolves at level 40 in Pokemon black and white
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Magikarp has the 7th lowest stats of any Pokemon and he can only learn 4 moves.