I haven't played OG Nemesis, so take this with a grain of salt... but Lockdown wasn't all that difficult to learn. The rulebook is pretty good and there are decent how to videos out there. From what I understand there are some additions that make Lockdown more complex, but if you're used to games of this complexity it's completely doable.
They're both expensive games, so I likely won't buy Nemesis, and don't really agree with buying the original for simplicity's sake prior to buying Lockdown (if you were leaning toward Lockdown). Of course if you know/think you want both at some point, go for it.