CS54...voyager, vario (kid/max/crowne/etc)...umm...basically if it looks like the same seat & mentions the 4 star crep rating on the box then it's the same seat LOL
We have 2 varios, DS1 is 8yrs now & still fits in it without a problem
he's about 128cm tall & umm, I actually have no idea what he weighs LOL last time I checked was around christmas & he was about 23kg back then.
They are a great seat, my kids say they are comfortable, no complaints of sore bottoms on long drives either
I have only seen pics of kids sitting in the hi-liner, it seems very narrow across the shoulder area compared to the vario. I don't go into baby stores very often though so have never had the chance to try either of my kids in one to find out for sure.
I also have the old version of the 'Urban II' seat you mentioned. DS1 used it from 2.5yrs old (when I didn't know better) until he was almost 6yrs old when DS2 was born. It's now our backup seat & still looks the same as when we bought it...except for a little sunfade on the top LOL
Very easy seat to use, just make sure you remember to use the sash guide