i never did internal on my roadie...but i have done 3 internal wirings on harleys and used 18 gauge...once used 16 and it was ok but real tight, just stagger your splices by at least an inch, he shrink your solder joints, and try to get your hands on the big heat shrink that can fit over all your wires....if your bars on already drilled make sure that you deburr the holes, and maybe open up the hole on bottom of bars a little more (i also bough all my wire to match colors so there was no confusion, costed a little but now have a whole heap of wire to use for future)

as for cables they are certainly very expensive, thats why i would suggest to get a little longer than you need so you only buy once ...also see if you can keep anything by just rerouting, i ended up not needing to change my throttle cable with my 16s on stock risers by moving things around....

also id say drop the return throttle cable you dont need it and it just another 20 or 30 bux to spend for a cable...you dont need to replace all your brake lines, just the top one that goes from the splitter to the MC