The person who will buy sell this big amount. He will not only deal on trust by just a mail.
Also show your bank asset proof.
If you will be having this much amount in your bank. It eill also be clear that you can buy.. but on bitcointalk you wont find a giant buyer like this. So,
Better way to buy from different exchanges and in 10-10btc partially u can achieve your target.
Thank you. Wish u all the best for buying and future growth from your buying.