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?
No, the bitcoin blockchain has never been successfully attacked with a 51% attack. There have been concerns over miners controlling a large share of hash power and thus launching an attack, but the Chinese clampdown on mining has helped to lessen the concentration of miners in China. Every day that bitcoin survives it gets stronger.