What was the most challenging part for you in building this emulator?
The PPU took 7 days, the CPU took 5 days, the APU took 1 day, and there's a spare day for everything else.
The CPU was easy to make: Its natural unit of output is a single CPU instruction. Even if there are many details, it's a straightforward process to compare an instruction-by-instruction log with a reference log until there are no differences.
The PPU was harder because the natural unit of output is an entire frame: It's not any easier to get the first pixel correct, than to get them all correct. So almost everything needs to be in place before you can validate its output.
1. Was this your first NES emulator?
2. Your first project in Rust?
The problem is I haven't really found any yet that helped me (admittedly, I have so far devoted just one Saturday morning to it.) Are there any specs about the hardware that you used, or that anyone else can share?
Everything you need is either on http://wiki.nesdev.com or on their forums. I know because that's what I did with my NES emulator - I explicitly didn't want to look at any source code and instead wanted to implement everything only based on the docs.
Granted, what's on the NESdev isn't always easy to grok, up to the point of being really confusing sometimes. What I've found really helped is gradually setting up a test suite based on various test ROMs I could find, which even allows you to implement some parts of the emulator TDD-style once you get the basics up and running.
http://tasvideos.org/EmulatorResources/NESAccuracyTests.html