The gearing for the Logo 800 is designed around a 380kv motor running on 14S (the 5035-380 comes with the kit). Using a 500kv motor will push head speeds too high. Remember 800 class helis with such large discs use lower head speeds than the average 700 class machine.
The full size 5.3 V-Bar will work fine. Although Mikado makes an "extreme" version for the heli, we have used the standard V-Bar on the 800 extensively, and it flies fine.
We also have been testing and are currently running a Scorp 160 on the 800, and are having no issues.
As for 12S vs. 14S. If you are a VERY mild sport flier, using lower head speeds and just "cruising around", you might find 12S to be satisfactory. But any pilot that has experience say with a 700 class machine, and flies any type of 3D aerobatics with a 700, will be disappointed with the machine on 12S. It would be like flying a 700 on 10S...which most guys would agree would be lacking in performance. No difference with running the 800 on 12S, instead of its designed 14S system.
Hope this helps,