Q1 - How many stretchies should I get? Nappy Lady said 4-10 size 1 and 10-15 size 2 but that was based on full time use
Your DC is already 5 weeks right? Might be worth trying to skip the size 1s if you can. Size 2 is supposed to fit from 8lbs/3.6kg if I remember correctly. Order a few to start with to see if you get on with them. My initial stash (of a different type of nappy, but that doesn't matter) was 15 nappies washing every other day, but I only started when DS was 11mo and I think a 5 week old still needs more nappy changes than that.
Q2 - How many wraps?
You can reuse wraps a few times during the same day (except if they get poo on them). So sometimes you'll just cycle through 2 wraps all day. Also when they come out the wash they only need about 15 mins to dry (or 5 mins on the radiator). We make do with 6 wraps (although will soon have 10 wraps when my shiny new Blueberries arrive...).
Q3 - How many boosters?
I don't use boosters in my stretchies apart from the snap in one that is already included with the nappy (although you'd take that out if you were putting it on a newborn, so as not to bulk up the nappy too much). In my AIOs (MioSolos, they are microfibre) I use one Little Lamb bamboo booster which helps them last up to 4 hours. 10 boosters should do you fine. Definitely recommend the LL ones, they come in two sizes.
Q4 - What are your favourite multisized AIO or pockets or similar for easy daytime use?
I don't actually love MioSolos it's just that I invested in them and stuck with them despite having some problems. They're good for DS because he's tall, but we've definitely had wicking around the legs at certain points (although not currently, probably because his legs are a bit more chunky now than they were). Also apparently despite the weight range they indicate, they are not a great fit until a bit later (Nappy Lady has some advice on her site).
I think especially with AIOs you really need to try them out (buy 2 or 3 of each you want to try, try and get them second hand, the Facebook nappy groups are great for this) as some work perfectly for some babies, but not for others. Really depends on your baby's shape! I tried some Bumgenius Freetime, which is supposed to be BTP but they were TINY on my DS (and he was only 18mo when I tried them so a loooong way off potty training yet). I also have some Smartipants which I actually really like, they do need boosting though.
Q5 - How many of Q4 should I get?
Depends on how you use them. If you want to use AIOs fulltime during the day and just use the two-parters for night time, then you'll need at least 15 I'd say. If you only want to use them for the change bag when out & about you could probably get away with 6-8.
Q6 - Your views on velcro vs poppers
Difficult one! I prefer velcro especially on BTP as it's just more adjustable, but it does start looking manky after a while and if you're unlucky some poo (or a bit of raisin..) gets stuck in it and you'll be picking it out with a nitcomb
. My popper nappies still look like new, but can't always get the best fit on them. Since starting to use reusables (11mo, DS is now 23mo) DS has changed shape so much from very skinny legs to much more chunky legs. Velcro definitely helps get a good fit if your baby has a big discrepancy between their tummy & leg size. My DS is still in vests so haven't had any problems with him undoing the velcro yet as he just doesn't have access.