So you are talking about a 50% attack, which is also executable on the Bitcoin chain and basically any crypto. It at first appears more tempting for Peercoin because it doesn't require investment in hardware like it does on Bitcoin. However, you do invest in the digital coins, hoping to sell them before you unleash your attack chain. This is similar to selling your Bitcoin hardware after attacking the chain, in that you can recover some of your investment and still carry out the attack. So on its face, 50% attacking Peercoin is similar to 50% attacking any crypto, in that it requires overcoming whatever network effect the coin has generated.
For the specific long-range nature of the attack you describe, it is important to realize that clients will not reorg beyond a certain depth (Peercoin has two types of checkpoints: 'synchronized', which is what we've been talking about, and 'hard', which is what I'm talking about now and something that Bitcoin also has). So what you describe will cause a fork between fresh chain downloads and old nodes. As the checkpoint server is an old node, it will not follow the attack chain and new users can follow the checkpoints to get on the old chain. Then there can be an emergency client update that specifically bans that fork, or something similar.
A 50% attack is indeed no joke.