Gotta go with coinbase on this one (assuming US citizen). Would have to be purchased over two days I believe.
Kraken if EUR-based
I use coinbase and I'm happy with them, but the OP should know that they have a $3000 USD per day maximum on buying. So it would take a lot of transactions to get all of your 61k bought and transferred to your private wallet.
Maybe he can use several exchanges at the same time, like coinbase, bitstamp and ANX. Also he need to have multiple bank accounts too.