In reply to Jackspratt:
I think it's safe to solo the volcanoes in Mexico - most people tend to climb a lower peak like Malinche, followed by Iztaccihuatl and then Orizaba.
Orizaba and Izta are glaciated. The glacier on Izta is tiny and there's no need to rope up. There's quite a lot of loose rock and route finding can be difficult, though. If you're climbing alone, I suggest that you climb over a weekend when there will be other parties of local climbers about...I haven't been able to find a decent map and RJ Secor's guide (Mexico's Volcanoes') is no longer accurate (due to volcanic activity, it was accurate when most recently re-printed in 2001).
Climbing Orizaba involves crossing the Jamapa glacier. Many people rope-up for it, but I have seen plenty climb it solo - there are very few crevasses (mostly tiny, you can stride over them), the terrain is pretty straightforward and there's aren't any route finding difficulties. It steepens to about 50deg near to the summit, though - if you slipped here you'd take a huge fall. It wouldn't be wise to climb solo after recent snowfall, as this might cover up the crevasses.
I wouldn't advise climbing any of the big volcanoes in Ecuador solo as they're heavily crevassed. The weather in Ecuador can be quite unstable, too.
The possibly exception to this would be Iliniza Norte - this is a rocky, scrambling peak with no glacier to cross and fairly straight forward route finding. I've climbed this solo a couple of times.
The volcanoes in southern Peru are mostly walk-up's…I've only been there once so I can't really offer much advice. I know that Chachani and El Misti are popular climbs with no technical difficulties and pretty easy access from Arequippa - you can climb them in a couple of days from Arequippa. I climbed Chachani about 10 years ago, solo. It's very high, but the terrain is straight forward.
Sajama is still on the 'to do' list - a good idea would be to head to Arequippa in Southern Peru first for some acclimatisation climbs, then cross into Bolivia and have a crack at Sajama. This would be an excellent trip.
If you're looking to do some research, I'd recommend that you get your hands on John Biggar's book…far and away the best general guide book available for the Andes.
You could start your research here:
Ecuador: www.gorillamountaineering.co.uk/south-america-expeditions/south-america/ecuadors-volcanoes.html
Mexico: www.gorillamountaineering.co.uk/south-america-expeditions/south-america/mexicos-volcanoes.html