I found it easier when starting out to build up what I needed gradually, as I got used to what worked for us, so if you don't have all you need now, it doesn't matter so much.

Smalls probably won't last you a long time. As for the others, how long they last depends on how they're cared for and how fast your child grows/how heavily she wets etc. The more often they're used, the less time they'll last.
Most people get a good length of time in mediums. OSFM didn't work well for us, so I can't really comment on those in particular as we stuck to sized AIOs -mostly Ittis. I think I had a rotation of about 20 that were just beginning to lose their elasticity after about a year's use. But that was at toddler age - younger babies go through more changes.
You might not find the OSFM a good fit while she is little, which would leave you with only the 8 smalls, not likely to be enough for a young baby. You might like to look into prefolds and a few covers for the early months while there are so many changes.

I personally like flushable liners, but it's not important to everyone, especially at the newborn stage.

It's so easy to get hooked isn't it? It might seem a bit overwhelming at first, but I found reading through older threads on these forums to be really helpful when I started - if you can find the time Well done on just jumping in there too, btw! Have fun.