What size prefolds? Do you have any indication from your dr about estimated size of baby? What size are the other prefolds you have?
I would definitely suggest pins or snappi to attach the diapers. I personally prefer pins, but I've noticed that most people tend to prefer snappi...
Do you have a surplus of receiving blankets, or can you get some really cheap? Cotton receiving blankets (the super thin ones) are excellent for flats, and that might be able to stretch how often you need to wash.
Wool is nice, at least for nights. I try to use it for night now that my DS is sleeping about 5 hours at night (yay!!!!) I don't have much, and almost all of mine is made from thrift-store sweaters...
If you are concerned about the cover situation, you might want to consider getting some dappi nylon covers. They are usually 6$ for a pack of 2 covers. They might be a pain once you have breastfed blowouts, but by the time ds was getting poop out of the prefold/flat for me, he was fitting into new flats (but not the ones leftover from his sister... Those are still too loose in the elastic for him even now).
If your flats are used, expect to not be able to use them for a bit. You might be pleasantly surprised. Ds fit into new flips at about 4 weeks, and at 9 weeks is still not fitting into flips that were used for 9 months... I don't know how much other brands relax, but the flips seem to relax badly.