The most important skill is good soldering.  If you have limited (or no) experience or are sloppy at it, then practice, practice, practice.  A cold solder joint somewhere could be impossible for a novice to troubleshoot.

The ability to read a schematic is very helpful.  While you can just try and put something together from a layout picture, you may miss exact what each part is suppose to be doing. 

Ceraitone's kits could with the circuit board already populated (capacitors resistors, etc already installed), though there are plenty of kits that just ship parts.  And there are NO instructions for Ceriatone kits, so it helps to know what you are doing. 

I recommend buying and reading this book.  It does a lot in explaining what each part in an amp is doing.  It even has a chapter on building a simple amp the author laid out (Victoria Amps offers a kit for it). 
http://www.amazon.com/Guitar-Amplifier-Handbook-Understanding-Amplifiers/dp/087930863X/ref=sr_1_3?ie=UTF8&s=books&qid=1276188618&sr=8-3

Amp kits that come with step-by-step instructions are available from Allen Amps Metro Amps, and Marsh Amps.  But once you get through a smaller simpler amp, you may be ready to tackle larger amps without them.

Troubleshooting usually requires some knowledge, so you may have to consult a local amp tech if the amp is powering up right.

Yes.