The Zenith is a nice tank, I think I discussed it on the other thread you joined, but I think it would be an excellent topper for a P3 if you want a drop in tank. Both coils would be fine, either the 0.8 or 1.6, the P3 would support either, and neither need super high wattage. Best of luck, as far as topfill, the Zenith's topfill system is easy to use and reliable. I like it. It's just a good all around tank and perhaps not my very favorite, which are my RTA tanks, but for a good, all around well-built reliable and easy to use topfill tanks, it's got some major advantages, including ease of use, long coil life, reliability, and it's built solidly. It has plenty going for it, for sure.
Anna