You have to be really careful with those CC/CV supplies if using to charge batteries directly. One wrong setting and bye-bye lipo! And it makes it harder if you have different pack "S" sizes. A true lipo charger has additional safeguards to detect cell count and memory profiles to store several settings. Convenience and safety comes at a price.
To set up one of those CC/CV supplies, you would enter in the max current to charge at (1C rate), and then a voltage limit of 4.2v x the number of S cells (for lipo) or 3.6v/cell for A123. This voltage setting is the most important and the one that can cause
major issues if you mess up.
Many/most computer power supplies have a 125/240v switch on the back panel to select what you have for mains.
And you have to remember that chargers have efficiency losses (typically around 80% efficient). You have to take this into account when figuring the current you need. If the charger has a 250w limit, that is 20.83A @ 12v. But, figuring in efficiency means you need 26A @ 12v. And since you generally don't want to run things at their max, I usually figure in another 10-15%. So, that's almost 31A @ 12v. Also, the 12v line will drop under those kinds of currents so the charger will try to pull
even more current to compensate for the lack of voltage. Being able to tweak the output to ~14.5v would reduce the current required substantially. But since it's hard to tell which PS unit will have the ability to tweak the output a little higher, I would shoot for a higher current PS, something like 35A.
This page can help select the right size PS for your needs.
Paralleling the output of a dual 12v rail supply may or may not work. I had one where if you did that, the supply got quite hot quite fast and shut down. IMO, it's best to get a single supply type. Here are just a few seemingly decent ones:
35A:
http://www.newegg.com/Product/Produc...82E16817182010
35A:
http://www.newegg.com/Product/Produc...82E16817148009
28A:
http://www.newegg.com/Product/Produc...82E16817101520
25A:
http://www.newegg.com/Product/Produc...82E16817170017