What bike are you going to use? (pics please)
I have built over 40 shifters with the sick bike kits and if you are not mechanically included you are better off going direct drive as a shift kit is not for the faint of heart to do right.
As mentioned in a previous post forget the 5G, it's a dual chain monster wide transfer case that sucks, forget a Stage III as well, the 4G however has been reliable and if you want to tackle the mechanics of mounting a shift kit they do make for a pretty sweet ride.
Again, builds like these are not for first timers, even with as many 2 and 4 stroke shifters as I have built it takes 30 some man hours, special tools, etc.
They are not cheap either, every one of those builds was $1,000 give or take $150 depending what needed doing and bike price.
For just a 4 mile commute you should be fine with a 4-stroke direct drive like this poor example of my work in my book as I didn't paint the tensioner or red plastic motor cover but you get the idea.
I get my kits from gasbike.net.
http://www.gasbike.net/bike-motor-kit-grubee-2011-skyhawk-4-g-t-belt-v.html
Under $400 delivered.