A media filter is great no doubt about it yet in my experience on any install done i've always placed them closer to the unit in either a pre-cut box or just a 4 inch opening on the sheet metal. The upside to placing it near the unit is that it wont make as much noise, it will just increase the static of the system. By placing it at a return grille its going to have a long run back to the unit and it will be much louder, you will most likely hear the suction of air. If you do install it I would definitely install it closer to the unit and just remember to remove the one in the filter grille assemble. I would really only replace it if someone has asthma, keep in mind the x4 filters are a little bit more costly. As for the light it depends your use, will it be for coil treatment or used as an air cleaner. The lights themselves are pricey as well as the replacement bulbs. If your using it for coil treatment i would hard wire it to run all the time. If your going to use it to treat the air I would wire it in series with a sail switch so when the blower comes on the light comes on. Check out Honeywell's website for the light depending on the size of the duct and then the choice of placing it in either the supply or return is upto you. Good luck