My questions are regarding the level of security endowed within Bitcoin.
Has there ever been a 51% on Bitcoin's network? I understand that as more miners enter the network, the greater the hash rate meaning the more difficult it is to perform an attack. This also means that it was much easier to perform the attack during the early years of Bitcoin, that's when it was most vulnerable. Was there any attack in the early years of Bitcoin and how much would it cost now to perform a 51% attack?
I don't think we had a 51% attack on bitcoin as such.
Once in 2014 a mining pool called Ghash.io reached above 50% of total mining power and as the community was concerned about the risks of a single pool having more than 50% of hashes, the pool Ghash.io voluntarily lowered it's bitcoin mining power below 40%.