[Ecky]

-Catalyst price seems to be in proportion to the amount of precious metals in them, which correlates to how well they work, and how long they'll last. A $250 unit may not actually be able to shut off the CEL, since the Insight has such low tolerances in emissions. My leaning would be to put an O2 fooler on instead even though it's (technically) illegal, since the car is still cleaner than 99.9% of vehicles on the road with a cat just degraded enough to set off a CEL. If it's found in a state with inspections, they generally just tell you to take it off and fix the cat properly.

-IMA battery would generally show the most gains in winter, yes. In a climate warm year-round you'd likely see little to no difference. The car still delivers great economy, of course.

The silver with 150k sounds most like mine was, and comes from the state with the least salt. The red by virtue of fewest miles would have the least exposure and wear on most parts. The 252k mile car could be a good choice depending on how clean it is, but remember that everything not replaced has that much more wear on it compared to the others - things like the seats, bushings, cables, etc.