It is easy so no worries there, just make sure to measure twice!
I borrowed the silicone sealant (solar seal was the brand) from a friend since he recommended it (used it only for the brace+tube), and went with caulk cord for the outside flare and the driver itself.
If you're going to brace it, which I'm not sure you'd need to since my sub weighed over 95lbs, you'll have to take into consideration the space that brace will take up. My brace was a board direct center, with a hole cut out as large as the front where the driver was inserted.
Killz is awesome primer, only had to wait about 6 hours til I was able to sand and apply another coat. I applied 3 coats before tossing it back in the living room.
Never worked with truck liner, but I'm sure others have. I know some have used it for sub enclosures, I know I wanted to but I just never got around to finishing it.