How much would a heater be for it? Its just an idea for me

I live in canada and we get cool wether so what ever i put in it would have to be ok.
Would gold fish not grow to big?
I would like 2 fish to go in it...
If i get the heater could i put guppies in it? What wuould be better show ones or the ones they sell for food(i think the later are harder..)? 66°F - 84°F (19°C - 29°C) is the temp for them right? Also what about platies? or minows?
Goldfish, Comet, Moor, Black Moor, Lionhead, Oranda, Pearl Scale, Ryukin, Panda what one would you recomend for it?
also how many galons would the barel be?
Sory for so many questions Im just thinking this through and found i had no idea lol.