In a previous post you were asking about pack size. If you can get in all in a 25L bag it will be under 5kg (unless you carry a lot of water - not advised). 30L gives you the opportunity to make it a bit heavy, and some people do fill 35L bags for an MM but they can't do much running.
IMHO, if you are going for a 1kg sleeping bag you're in the 30L bag range (and probably 6kg). As an infrequent MMer that is probably not a bad thing, you won't have to buy all the kit just to get it in your pack. Like you say no point shelling out on expensive kit if it's not going to get used much.

