Interesting. Inspired by you, I entered "FMCW" and "ISM" as search terms in google, and I found a few companies, too. Including one which mentioned up to 250 MHz of bandwidth in the 24 GHz ISM band.
I imagine that the front-ends would work fine. AIUI, these are radars which measure the range from the top of the tank to the fluid surface. So it would be the European equivalent of the FCC that you would have to worry about if you used something like this outside of an RF-tight box. (Unless, as above, they were operating in an unlicensed portion of the spectrum.)
Not at all. The wavelengths are pretty small, so you don't need a big transducer to get directionality. Now that I think about your range, though, it may be at the outer limit of what ultrasound can do. You said
10 meters, right? So you would need to stick with low frequencies ultrasound. No more than 40 kHz, probably.It sounds like an FMCW radar could actually work, but have you considered Lidar? I don't really know anything about it, but I think the successful DARPA grand challenge vehicles used it.
--Mac