I like both Duracoat and the ol Krylon rattle can because I like to DIY...and either is simple (Duracoat is easily applied with a cheap Preval sprayer if you don't have a small HVLP gun).
Nop opinion on the optic as I'm target only, but I've read some great stuff on the Trigicon and their illuminated dot system (I think the dot itself is like .7 moa) is as high tech as it gets.
18" or 20" is going to be pretty much a matter of preference, you'll get acceptable velocity for typical hunting range applications from either, and the .308 based calibers are relatively insensitive to barrel length compared to some others. Me, I'd go with 20" unless there's a reason you'd benefit from the additional handling ease of the 18".
Threading is a no-brainer, since you're already chopping the barrel and assuming you're planning on adding a can or flash hider/brake. If it's a brake or flash hider, best to have it in hand to give to the smith so that the threading can be perfectly done, and he can turn the brake/FH if necessary to match the barrel contour for the best appearance.
Love the 7-08!!