You can't really go wrong with a bigger heater especially for a spare water container. If you main tank heater ever failed, you would have a backup. Plus, the price from a 150 to a 200 is normally not a big deal. A 150 would do fine though.
Pumps are normally meant for sump setups. Powerheads are for intank water circulation. Pumps can be used for this via a closed loop but most just use PH
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,