Where to buy NEM and do I have to use my BTC to make the transaction?
you can buy nemstake(that will later be redeemed for real nem) on the nxt asset exchange… you will need to transfer your nemstake too which "secureAE" doesnt facilitate atm so going with the native client is best i think…
you can download the client here… https://nxtforum.org/nrs-releases/nrs-v1-2-9/
you will have to buy nxt and send it to the nxt address that you create… be very carefull with your password… it has to be very strong and dont lose it… i would suggest to use the built in password generator and use keepass to save it. log in and out a few times to be sure… then go to "asset exchange" > enter asset ID: 12465186738101000735 > then you can place your order.
Thank you and another "applaud" for you my friend! 
No problem and Thank you very much.