My employer, an RBOC, uses the HDSL products of Adtran.
HTU-C comes in at least three form factors; 220, 3192 and DDM+ for the central office end. The 220 is the largest, the other two are approximately the same smaller size. I don't know if an equipment enclosure for one or two of the 3192 is available; the 19 or 23 inch relay rack holding 14 (220) or
28 (DDM+) HTU-Cs are installed in my CO. They expect 48 to 56 Volts DC in operation, and power the HTU-R over the twisted pair. Each end expects a DSX-1 connection in and out. You can search their site on this CLEI: T1L3XBZAAA. Whether you'd need a CSU/DSU at each end I couldn't say.
You've probably already seen an HTU-R in a telephone equipment room on site. Enclosures for two Plugs are common. T1L3W7WAAA
Adtran has circuit design software for download. You can readily count on
12,000 feet using 26 gage copper twisted pair, with no taps, using the two-wire models. Four-wire gets more distance.