It can be soooo confusing, even for a veteran! Everything has changed a lot even since I cd'd my first 10 years ago!
Ok, so Sandy's vs One-Size. Basically comes down to personal preference as both are excellent high-quality diapers. Each has their own advantages and disadvantages. Basically it comes down to trimness vs frugality. The One-Size is bulkier for a few months but it is also more economical. Now, obviously that's a pretty big oversimplification but it's the gist of it. It's all basically personal preference, you can look up past discussions for more detail. Let me know if you can't find any and I/we can elaborate... You can also get a trial pack with a diaper, cover and liner for under $20 (or a little more if you get more expensive diapers or fabrics) including shipping and taxes so you can try/see for yourself for a pretty reasonable price. On the website is the cotton One-Size and Airflow trial pack but you can call to get a different diaper, fabric or cover. If you live near the ME factory in St-Catherines you can also check everything out in person or you could also contact them to inquire if there's a retail CD store near you that carries ME products - there are quite a few throughout Canada. Plus you can return anything that's unwashed and unused. Or you can sell on the buy/sell forum to recoup a good part of your money.
How many diapers... Figure a newborn goes through about 10-12 diapers a day and that diminishes to about 6-8 for older toddlers. So using those figures, think about how often you want to do laundry and if you'll be mostly using your dryer or line-drying (line-drying takes longer, especially in winter, so you have to give yourself a few extras to last through till the clean ones are dry). A good average is 24 but can really vary from 18 -36, depending on individual habits, preferences and perhaps budget.
AIOs. Most people who use them do so mostly for quick changes when outside the home or for reluctant sitters who may be confused by seperate diapers/covers. They're more expensive than seperate diapers/covers, take longer to dry and may not offer the same longevity (since the cover part is washed in hot water after every use, whereas a seperate cover can be reused several times and washed in cold/warm). All that said, they also offer the advantage of quicker changes and the ultimate in simplicity and this is important to some people. It's not very common to use them exclusively but some do and love it so it's basically up to you to see if it's worth it for you. There are more in-depth discussions on this topic too if you're interested...
Stay-Dry... Basically personal preference, they have advantages and disadvantages.
Liners... There are basically three types of liners and each is pretty different from the other. The disposable liners you mention help in the cleanup of a poopy diaper and may help prevent stains. But they don't offer any additional absorbancy like you'd need for nighttime for example. Extra absorbancy liners (aka doublers on some forums) are made to add to a diaper to add absorbancy, usually for nighttime. Stay-dry liners are made to help keep baby's skin drier, can also help in poopy diaper cleanup and help prevent stains (you can get commercially made ones or make your own by cutting polyester fleece to the size/shape you want). On their own stay-dry liners don't offer additional absorbancy but they can sometimes be sewn on top of an extra-absorbancy liner so that you have both in the same piece.
Detergents... Soap nuts are definetly not necessary at all. My first recomendation would be to keep things simple and just use whatever you use for your regular laundry. The vast majority of people do fine with regular mainstream detergents (no need for even any special 'baby' detergents). The only thing you need to remember is to stay away from bleach and fabric softener (and any detergents that may have those built-in). Plus make sure you use 1/4 - 1/2 (at most!) of the 'recomended' amount. Modern detergents (and this has likely changed since you diapered your first) contain additives that are actually made to stay in fabric like stain guards and UV brighteners (these stay in your clothing and reflect light to make clothes appear whiter and brighter). Over time they can build-up in your diapers and cause nasty odors or rashes. There are no other true 'rules' and while soap nuts and special detergents are great and often slightly more natural or eco-friendly, the whole detergent issue can be awfully confusing. Especially since one person will swear by one certain detergent and then the next person will be swearing AT this same detergent, there's no one routine or detergent that works for everyone. IF you have problems with your regular detergent then you can consider switching but honestly I wouldn't worry about it right now.
HTH and good luck with the new baby!