Maz's covers are a great fit on prefolds & flats.

I would get a pack or 2 (I got 2 for one bub) of terry flats & maybe one of flanny flats (haven't used myself) for back up. They dry the quickest & if it rains for weeks straight even your prefolds can take longer than a day to dry.

Go to the buy & sell your mcn's on facebook page. You'll be able to pick up cheap covers & nappies, mostly second hand, but they give you a description of the condition of them
You'll be able to find out what works best without spending too much

I have about 11 itti's, 3 swaddlebees, 3 ebay cheapies, 2 daddle doos & 2 other wahms that I use regularly, not including nights. So thats 21, at 3 months old just for day use.
I used prefolds & flats when she was younger & being changed every hour or 2 night & day. I wash every 2 - 3 days.

I'd start with them (flats etc) til the bubs are here. It takes a bit to work out which nappies fit the best to keep in the poo plosions