Joe,
<<2. I'm looking for a combination (if it exists) 12V / 24V power
source / battery charger to test the electrical system and later use as
a battery charger.  I was asked by a supplier what Amp's I need.  Can
anyone answer that question?>>
For any electrical testing during construction I used a huge 90Ah boat battery that I bought for only $43.  It didn't need charging very often.    Now for maintenance, I use a Schauer battery charger that has an adjustable end voltage.  I connect it to the aircraft battery via a permanently installed pig-tail.  I know they make a 12 and a 24 volt version - don't know about a combination.
Chris Zavatson