Converting from square spline to external bearings, what do I need to know?
You actually guessed the only thing that you really need to watch out for: the BB shell width. And you don't even necessarily need that until you're actually putting it all together. Almost all BB shells are 68 or 73 mm. Most of the external BB's come built for a 73 mm shell and include a spacer for a 68 mm shell. 

The odds are really really good that the threads and diameter coform to the British I.S.O. standard. If you have reason to think that your bike is non-standard, I'd recommend taking it to a shop and they can tell you what you need. 

Unless your mountiain bike is a single speed, you don't need to worry about chainline. They're pretty standard and a derailleur adjustment will take care of any deviation from your current setup. If it is single speed, the BB isn't where you'll be looking to get that right. It's not like a cartridge-style BB where you can just get a different length spindle. Instead, you move the chain rings to the inner or outer side of the spider and/or mess with spacers in the rear wheel.