I don't know if you should really be including ALL sizes of covers in one shot- what if their baby doesn't do well with a certain brand or material of cover? I'd also include some wrap style covers- some parents will NOT stick with CDs if it means fussing with a snappi and a cover.
Wool CAN be expensive, but it doesn't have to be. It can be the only cover option for babies prone to rashes,and it's the only thing I've ever had success with overnight. I'd suggest including a couple of wool covers for nighttime use (skip the longies- wool covers can be worn under PJs)
I agree with providing both snappis and pins.
Prefolds seem to be pretty popular, but I personally preferred flats- they wash and dry faster (a key factor if you're using a laundromat, line drying diapers, or have a tiny washer and dryer in your apt.) They're also more versatile in terms of sizing- I used them exclusively for my son for a few weeks after he outgrew the dipes I made him, before I had time to sew some bigger ones, and they fold up as small as needed for newborns.
When I CD'd my first, I had about 5 dozen flats, 6 vinyl pull on pants, about 2 dozen baby washcloths, and a dozen pins. I used plastic trash bags to line my diaper pail, plastic grocery bags as wetbags, and ziplock bags to carry wipes. As she grew, I bought bigger covers. When I started CDing with 2 babies (after a hiatus with 1, then 2 in sposies) I bought another 4 dozen diapers (2 dozen flat and 2 dozen prefold) and more covers, then bought more covers as they grew.
With ds, I made contoured diapers out of an old flannel sheet, got some covers as an RAOK, then had fun sewing fitted dipes for him. I think I spent more on diaper sewing materials since ds was born than I spent on all diapering supplies combined for the girls!! See what the internet can make you do!?!?!?! In all seriousness, most of my diaper fabric was an investment in a business that hasn't quite gotten off the ground yet (and may never do so

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