Most taxi drivers charge 20 soles from the airport to Miraflores. Pre-paid is usually private transfer and is going to run a bit more. Its a half hour ride down the beautiful coast.

Yes there is currency exchange/withdrawl before you exit security at fair prices.

Miraflores is great during the night. Walked along Larcomar all the way to Parque del Amor during the night felt safe. Not sure what you mean by "central area." We stayed in miraflores and I woudn't stay anywhere else. Its like a completely different city in Miraflores. Entrance to the club at Larcomar runs 100 soles... LOL!!!