The Enigma Black Heavy Grail is my favorite. Using stock HHKB internals and 55g OEM domes. I personally just love the way Topre feels.
Topre is incredibly smooth and has the best key feel in my opinion. I have many other boards (MX, Alps, Buckling Spring, Space Invaders) but I always come back to Topre.
The aftermarket cases get pretty expensive, but the boards themselves are obtainable. The Fc660c and HHKB are pretty widely available. You could check out SHO cases.
It’s worth the price tag to me. They are expensive but they also hold their value pretty well so if you’re not a fan you can recoup most of your money.
I'm absolutely convinced that everyone who professes their love for some flavour of mechanical switch keyboard is someone who hasn't tried Topre / electrostatic-capacitive switches yet.