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Why is the Keling KL23H2100-35-4B 381 the best Stepper motor for the G540 right now?

Stepper motors have a specific voltage at which they perform best. You want that Voltage to be at or very-close-to the G540 maximum of 50V.

495= 85V
425= 84V
387= 65V
381= 54V

What PSU Amperage for three 3.5A 381s?

KL-350-48 7.3A

Unless you are going to be running two 381s, and a pair of SLAVED 381s (Which would require 10A PSU.) a 7.3A PSU will be fine. (Adjust it to 50V with all motors powered up.) You can get THAT a little cheaper ($53) HERE:

I believe, that John Keling might match that price if you buy everything else from him.

Why not just use the motors that come WITH cables from CNC ROUTER PARTS? These are 65V motors. The supplied cable is only one foot long, so you HAVE to buy the $16 cable extension, which makes the final motor cost $68.50. If you can solder, you can save a bunch of money and get more speed with 381s.

Where can I get a decent parallel port computer for cheap?
I have purchased several of these in the past 2 years with never a problem. They are refurbished and come with 1 YR warranty: http://www.tigerdirect.com/applications/SearchTools/search.asp?keywords=off+lease+desktop

Should I ever take a stepper motor apart?

Not unless you want to weaken it permanently:

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