Dunno why people are moaning about not having any 100hp-specific ones. A lot of dampers are common throughout ranges unless there are significant differences in engine size. Impreza's for instance shared dampers from lowly 1.6's up to higher spec turbo cars, they did however change the springs to lower the cars and of course to make them stiffer. I really don't see how the Bilstein's with stock springs or even with lowered springs could be anything less than a big improvement on the standard setup.
I had some Bilstein's on my 131 and although they weren't adjustable they were (so my dad says) far more damped and much better quality than the standard dampers it came with.
Anyhoo doing what sensible people do and browsing on the Bilstein online catalogue, the part numbers for the 1.1,1.2 and 1.4 dampers are exactly the same so no problems there. If you were feeling adventurous you could try to fit the coilovers Bilstein make for the 500 but there are no guarantees that they'd fit of course
Personally unless you're actually going to adjust the dampers all the time I'd stick with the Billies as they're cheaper, Bilstein aren't monkeys and you'll probably never use the adjustability anyway. My dad has Koni's on his 504 on the softest setting (always have been) and they're uber-plush and great on rough roads, he's also got some Bilstein's on my mums Saab 900i and it's handles really well and is very progressive