As a newbie to AR who has just completed homework but not implemented i.e. assembled or shot . . . . my 2 cents worth
The attraction of the AR is that you can "assemble" a rifle to your own requirements with two caveats, the limitations of the platform within the two "sizes" AR-10 (.308 boltface) and AR15 (.223 and a few in between boltfaces) and the limitations of the NZ supply chain i.e. if you want a really special barrel you'll have to wait (6 months or so maybe) IF a supplier will order it for you.
I carefully priced the online offerings of both assembled and build your own and there's a very small saving to build your own, however DIY is the path to exactly what you want at a modest cost. From my look at it - it seems the retail margin is pretty modest, my local gun shop has the new Ruger AR15 at $1650.
Because its a "parts flying in formation" thing its great to start out modest and add upgrades down the track e.g. trigger
As far as resale goes there are a lot of dreamers on TM who try and sell at nearly retail. However if you dig a bit deeper on expired listings, I'd say a $1400 new one will bring $1000-$1100 and a $1800 new will get $1400 - not too bad by rifle resale standards . All rifles seem a bit slow on TM at the moment. Custom assemblies seem to take a bit bigger hit (unless you know exactly what other people want) BUT the individual components seem to sell really well if you strip it, barrels bring $300 plus and they are $350-400 new.