Hi Jeff,
IMHO, the Lyra Helikon would not be overkill. It is a very nice cartridge, and worked well with my JMW 10.5 tonearm. No tracking problems at all. The Lyra is very different from the Koetsu and the Benz though. The Koetsu and the Benz's are warmer, fuller sounding. The Lyra has more air, speed and transparency. Of the three that you have listed, I would prefer the Helikon, unless your system is on the bright side and needs the added warmth.
As for buying used, I would not recommend this when buying a cartridge. I have bought many used components in my life, but none have bought me more regrets than buying used cartridges. There are too many variables. How accurately the seller tracks the hours used is a big one. Also how was the cartridge setup and maintained? How old is it? Even with low hours a cartridge's suspension can go bad from sitting on a shelf for too long. IMHO, I would never recommend buying a used phono cartridge. You would be better suited lowering your model and buying new. For example, buy a new Lyra Dorian or Argo instead of buying a used Helikon. That's just my $0.02.
Cheers,
John