Unless the ambient is below your desired dispensing temperature you don't need a heater.
If the keezer is sitting where protracted cold spells happen, you can use something as simple as a light bulb in a cheap ceramic base, or use a Lasco "personal heater" or similarly low wattage heater. In either case, you need a dual-stage temperature controller to keep compressor and heater in sync.
Back to your first concerns, I literally posted this a minute ago, but it applies in your case...
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Start at the beginning: use
our favorite carbonation table, find your beer temperature on the Y-axis, scan across that row to find the level of carbonation you desire expressed in "volumes of CO2", where 2.4 volumes is about middle of the road for most ales, then run up that column to find the CO2 pressure to use that will - in the fullness of time - reach equilibrium at the desired carbonation level.
You can carbonate cold or warm, doesn't matter as long as you use the appropriate CO2 pressure.
When you go to dispense the beer, the combination of temperature and dispensing pressure must result in the same level of carbonation. For example, you carbonate to 2.4 volumes at 65°F and 27 psi. Then you stick the keg in a kegerator set for 40°F. You should set the dispensing pressure to 11 psi (see the table).
Finally, to assure a good pour, your beer lines should be tuned to handle at least 11 psi. See
the only beer line length calculator worth using to understand the physics, or simply use 1 foot of 3/16" ID beer line per PSI (in this case, 11 feet would do it).
Cheers!
