Yes, this is the most important.

A private key is just a random generated number. This number is then hashed using SHA256.

But you can hash words, phrases, small numbers and this will generate valid private keys. Just put these hashes in bitaddress.org and you will get the public keys too.

If you want to find a collision, the best way is to try to find poorly generated private keys.

If you try to hash some words like "wallet" "satoshi" "bitcoin" you will find some addresses which doesn't have balances now, but they used to have. I tried many already, but manually. Never had any luck.

Maybe if you can make a program that can try millions of inputs per second, hash those inputs and check the balance of them, you may have some luck.

You will only find poorly generated private keys this way.

Edit: I read a post by butka few months ago about this, couldn´t find it now