You can pretty much experience any type of weather in the Drakensberg, any time of year. For this reason there is literally no difference between my simmer and winter packs, except that in summer I carry a tent (I don’t like getting wet) and I buvvy in winter. Oh, and if rain is predicted I might pack some rainpants.
Whatever bag you get, make sure it can go down to around -8 deg C and lower. Coldest I’ve experienced was -14 in a tent. I use a TNF Blue Kazoo. At 1,4kg for the 3xtra length bag, it’s rated at -10. Your cheaper (but heavier) alternative is an Icebreaker (First Ascent). Decent, but warm (-8). Started hiking with the Icebreaker in the berg, nothing wrong with it.
You can invrease your warmth levrl by sleeping in a bivvy bag, or in a tent, especially if you share a tent. More BTU will make up to a 5deg difference in average, I’ve read. Bivvy bag keeps wind out and as its a small space you can heat up the air nicely.
Having a cold night is not fun. Besides being dsngerous, it will leave you fatigued. Take a warm bag.