has some consumer stuff, that includes joints for the corners. Other companies have the same. Get whatever's cheapest if it looks OK.
As for cable run length, it depends on the length and the gauge. I swapped 75' of 20 gauge for 50' of 14 gauge, and it made an audible difference
. I admit 75' of 20 gauge is an extreme example, but I wouldn't run 18 gauge for 50' either. That's based on our calculations here and based on other recommendations
. Actually, those recommendations aren't even the most conservative. That table suggests 14 gauge, but others with more conservative assumptions would recommend at least 12 gauge for 50'.
P.S. If you get 14 gauge without excessive plastic covering, it is about the thickest wire you can easily put in the smallest Wiremold wire conduits. You can put 12 gauge in it too, but there may be lots of friction so sliding the wire around in the conduit for adjustments won't be easy.
I mention this because the smallest conduits are the most inconspicous, and they're also the cheapest.