First of all, welcome to the world of case modding. This forum is an absolute wealth of info and exchange of ideas.
Most of the methodology I implemented into my 2nd G5 was also implemented in my first G5 case build. Included here
are instructions for how I accomplished final presentation. In both my G5 builds I chose to use a dropped PSU rather than to dissect a sealed unit. There's mixed opinions on the PSU subject on this forum, but I chose to follow this route because it very effectively provided the clean look I wanted without voiding the PSU warranty or creating any risk of a potential electrical short. Furthermore, my home insurance policy contains language about modified electrical equipment beyond manufactured specifications, its just something I chose not to mess with.
I purchased my materials from a a local MetalMart shop here. I keep the protective film on all the plating material until the very last step of peeling it off. Measurements will be specific to your build, so I cannot provide those to you. I used a dremel 3000 and #426 reinforced cutting wheels for all my cuts. I recommend practicing a little before you begin, and always measure twice, cut once.
Planning your cable management strategy is the FIRST step. Knowing exactly where EVERY cable will be routed is essential to a clean and polished final appearance. The G5 PSU design graciously gives you 1/2" of clearance against the rear case that allows routing all your power cables through the side of it and up into the case while remaining hidden behind a cover plate as shown in my G5 builds. This 1/2" of space is a huge gift for effective cable management.
You're decision to locally purchase a complete G5 is a good one. There's plenty out there, so be patient and get a perfect unit if you can. Be aware that the late 2005 model has a different front panel PCB than it's predecessors that will need replacement if you buy that model. My 2nd G5 was as such.
Good Luck as you proceed!