I have an unhealthy obsession with Thomastik Jazz flats on several of my basses, including one fretless. They don't do that smooth/dull thing like a lot of flats, they are far more aggressive and mid-rangey. Here is my green bass with Thomastik flats:
And here is the fretless with the Thomastiks in a solo context:
They aren't cheap in the US, but I'm thinking you might get a better deal in Euroland. The good news is, if you like them, they will sound the same for years. I play the green Nordy constantly, so far I have about 18 months and 24082048524 hours on those strings, still sound fantastic. Extremely low tension, and for me that's good, but can be a turnoff for others.
Rounds on a fretless aren't the end of the world, although I'd probably stick with nickel plated strings instead of steels if you're concerned about chewing up the fretboard. I have a set of DR Sunbeams on my other fretless, they work fine but really I only went with them because I had a set laying about, and that's not one of my main basses anyhow.
D'Addario Half Wounds are very fucking cool on fretless, I think those are somewhat similar to the GHS Pressurewounds, although I was just told the GHS are brighter (yet die quicker).
For a nice cheap alternative flatwound, D'Addario Chromes are nice. They are quite bright for flats, and die very gradually. After a few years they will be pretty thuddy sounding, but they do stay lively much longer than typical flats.