Post to help you understand heat:
I'm trying to dial in my exposure protection for winter Lake Tahoe diving. It gets down to (and is currently) 39F, pretty much from the surface on down. I've taken a number of steps to boost my thermal protection and have successfully done 90 minute dives at this temp without getting too chilly...
scubaboard.com
I am of the opinion that combined heat + light into a single canister is a relatively poor idea and much prefer separate canisters. In terms of light, UWLD40>Focus2>LM32VF>Dive Rite EX35. If you do decide to combine the heat + light, the UWLD platform allows for this easily. The Light Monkey, on the other hand, does not, nor does the Focus2 or EX35. What are your target dives? Durations and temperatures? $1700 US or CAD?
If you're wanting to include heat, I'd suggest a bare minimum of 20Ah. The PDF in the above post will help you with the math.