The H in 6x12 is no longer a factory configuration. I got a killer deal on the one used for this review, in the low $2Ks. Today, the H25 (successor model) is €1,629, the 6x12 revolving section is €753, a 6x12 film holder is €1,365, the 58mm VF mask is €51, and the 58XL with bayonet, helicoid and center filter is €2,348. Needless to say, a Silvestri is something you would want to find used and as complete as possible!
Columbus Camera Group had a set up on that auction site for $3,250 with two lenses, 6x9 groundglass and roll back, 6x9 Polaroid, and 4x5 groundglass/film holder. Nice setup. You would have do do a lot to convert it to 6x12, since none of the parts other than the lens unit and body are the same. But you could conceivably use a Horseman 4x5-6x12 back and just get the VF mask.