The tallest Volcano on the South West Antiplano of Bolivia, Uturuncu sits quietly above the hills behind Quentano Chico on the road from Laguna Verde to Tupiza. It is somewhat "out of the way" and as is the case with most of the peaks on the antiplano, a bit hard to get to. Possibly a car could be hired in Tupiza. You could include this on a custom antiplano/Salar de Uyuni tour, and there is a bus and Camions that go to Quentano Chico, but you'd have to hike in, which would involve camping.
Assuming you can get to Quetano Chico (an uneventful mining and lama herding town west of the VERY spectacular Cordillera de Lipez, Uturuncu is not in this Cordillera), there is a road that goes to an old sulphur mine on the mountain. Ask anyone, the road winds out of town southwards and is very probably the highest motorable road IN THE WORLD. It goes up to 5,900 meters but the last parts are often drifted over with snow and not passable. Warning - Roads in this part of Bolivia, and especially this one to Tupiza, are pretty rough and can become impassable when wet.
No red tape, just the usual difficulty traveling on rough roads with no infastructure in a remote part of the world.
Year round, summer (Dec-Mar) is the wet season, but that doesn't mean much on the high antiplano. Colder in winter.
You can do this about anywhere but the refugios are so cheap, why put yourself through this. Cough up the one to three bucks for a bed and a roof.
I'm sorry, as with most mountains, you will have to check locally. Ofcourse we all know weather can change quickly and very dramaticlly at high altitudes.