Not OP either, just somebody who used slings/helped to run a sling meet for a while:
IME, stretchy wrap slings of any kind including cheap from amazon and home made are all exactly the same and there's no inherent value of Moby etc. However I've seen some cheap (often Chinese) ones on amazon recently which get reviews stating that they are too thin/short to be used for any carries so I'd watch out for those reviews on the super cheap ones or stick to ones with good reviews or which are made in the EU. I agree with PP that if you're looking at anything with buckles or stitching, stick to well known brands, because you REALLY don't want a stitch or buckle to fail. Be wary of fakes too - you can find this when a well known brand is suspiciously cheap. Sometimes checking their website for approved stockists is a good way to double check.
Stretchy wraps are really great for this reason as you can get them so cheaply and they suit almost all newborns. I always recommend people get a stretchy wrap before their baby is born and then look for other options once the baby has arrived, either when they are little if they want other options ASAP, or when they outgrow the stretchy wrap, it doesn't really matter.
I don't think there's any specific age you should take a newborn to a sling meet, just go when you feel comfortable.
Different techniques for breastfeeding in different slings. Personally I never managed it unfortunately. I did have a consult with a nice lady as part of training for the group who showed us how to get a good breastfeeding position in a ring sling but I didn't have a small baby at the time so couldn't try it out for real, and I've probably forgotten it now I'm pregnant. I do remember the ring sling being really unintuitive and hard to learn from pictures/videos but easier in person. Ask somebody IRL who has done it to show you how they do.
Newborns can be carried in a cradle hold or an upright hold in any carrier although structured carriers can be problematic for them as they are more designed for older babies, even with a newborn insert (the newborn inserts are also typically very hot which I worry about with tiny ones). You CAN use a non stretchy wrap for a newborn but it's harder to get the hang of. Stretchy wrap/caboo seems the easiest especially if you have no access to IRL support. Don't be intimidated by stretchy wraps, they look scary but they are so simple and easy.
For tall toddlers the Boba 4g is fantastic as it has a really tall body plus leg rests! If you're petite and particularly slim it's important to check the waistband size for SSCs because they need to be able to rest on your hips. If they don't tighten enough you'll have weight on your shoulders which can be really uncomfortable. There are some waistband comparisons online. OTOH if you/OH have a large waist you'll want to check the maximum waistband size and see if your chosen carrier has the option of an extender.
SSCs are a bit like jeans in that what suits one person won't necessarily be comfortable for another. Wraps and Mei Tais are much more customisable to any body shape but are harder to use.
We had plenty of bottle feeding mums at our sling meet and it wasn't an issue. Though there was a large crossover between LLL (breastfeeding group) crowd and sling meet crowd. I suppose unintentionally if you get the sense everyone else knows each other it might feel unwelcoming? I hope that wouldn't be the case, though.