If the install is done properly, you will not necessarily need a "coil thingy" or a capacitor. Make sure you have proper sized power and ground cables. Depending on how old your car is, you might want to investigate upgrading/replacing the factory ground and power cables. Things like battery to chassis, battery to fuse panel, alternator to battey, etc. Replacing these old/too small/corroded/oxidized cables will be an improvent to start with, and if you go up a size in cable (suggested) you will be even better off.

I did not ready the whole thread you listed above, but at first glace, that box seems to be built fairly well.