I just finished my first build, so the first things that come to mind are:
Chainlength won't be a problem (provided your derailleur capacity is correct for your drivetrain).
Cabling, in particular cutting cable housing to correct length, is indeed one of the 'hard' parts of a build. I would suggest:
1) Go very slowly.
2) Consider VERY carefully your desired cable routing, handlebar/stem height, and shifter placement on the bars before you go to cut or route any cables.
3) Cut longer than you think you'll need, you can always trim it shorter.
2) Get a Park cable cutter and a file, or a dremel.
Other than that, none of it will really be *that* hard, just be patient and read every instruction 17 times