The price listed above is in EU, it runs about $6300 US for the kit. Their price also includes the VAT. I think the VAT(usually a hefty %) is excluded for people ordering from the USA.
Madisound also carries a Linkwitz LX521 "kit" of sorts for considerably less money.. I appears all the pieces are there in separate sub-kits(except power).
I came very close to committing to the Linkwitz Orions years ago(2010-ish), but I shied away at the required power options. The current LX521's still require a true commitment (the build, crossover, non-std power..), but it's much easier. I expect the path to an up and running pair of the 521s is well worth the effort. I ran across this build-blog recently on the 521s. While a fair amount of work, the process looks pretty straight-forward.
https://www.afterness.com/audio/lx521.html
At the time(2012) I did pick up a kit to build GR Research's Super V. Long story, but I'm now, finally, building them up. I have the cabinets built(from scratch, no flatpack) and I'm approaching the paint-finishing stage. I'll run them with a Don Sachs pre and power amp(speakers have 97 dB sens, 8 ohms).
Hopefully the Super Vs will be my forever set of speakers. If not, a pair of 521s would be on my short list. It's nice that since Siegfried has passed, his speakers continue to be available. Quite a legacy.