With the short barreled Handi's, the 45-120 will gain you nothing in velocity. But you'll be able to lower your operating pressures a bit when using smokeless and have the near same velocities of the 45-70.
To utilize the big case of the 45-120, you need lots of barrel to make use of either - smokeless or blackpowder. With a short barrel and a stout load of powder, much of it will burn past the muzzle. If you short charge with smokeless in the 120 case, you get some horrible accuracy issues, and inconsistent ignition problems as well since the powder can settle far from the primer.
Cases can be had for around $2ea give or take - yes, they are a bit pricey compared to others. What you may have a problem with is finding a press that has enough stroke to load them without having to back the die out to insert a bullet or remove a loaded cartridge from the shell holder. Theres only a few presses with enough clearance to load the big monster 3.25" case, plus have room for the bullet. Buying loaded 45-120 ammo will run you into the poor house within a couple boxes. With such steep prices you will make out on buying an entire loading setup in a real short time period, unlike trying to compete with bulk 9mm stuff. You can save dollars per cartridge loading yourself, versus fractions of a cent to a few pennies for bulk mil-surp ammo for one's getto blaster.
As for "obsolete" legal status - no. The 45-120 ammo is still in production in normal commercial channels - internet.
