temp controlled fermentation will probably provide you with the best improvement in brewing results.
but a brewfridge is probably the easiest/cheapest solution for that
a nice SS brewbucket will last a lifetime and with careful fridge selection can fit inside a standard brewfridge.
the cold box cold sink idea floated in your link will require daily or more regular maintenance, most folk use maxi chillers with a python/recirc pump with laarger conical and SS vessels that cant squeeze into a fridge.
and with corny kegs, the attach to keg tap is about as useful as a chocolate teapot, in order to pour anything but jugs of foam you need the keg pressure dropping to nominal. Not forgetting the crucial aspect keg or beer temperature plays in maintaining and retaining condition.
speaking from experience buying into corny kegs before fully realising the costs and demands its worthwhile doing a bit of homework on the subject, the kit linked to is very expensive for a keg(ã35) regulator (ã30) useless tap and the small nonstandard bottle will be a pita and probably expensive to refill.
so in short Neither, invest in a brewfridge first, and then if it will fit in perhaps consider a nice ss bucket tho 33l stockpots are a lot cheaper too
