if you're only going to buy one carrier and want it to last into toddlerhood and you've got back issues, i'd suggest either a woven wrap (like a didymos) or a mei tai.
stretchy wraps like the moby are nice in the very early days, but aren't useful long term (especially if you have a heavy baby). and yeah, it's going to be hot in the summer IMO. if you're thinking about a ring sling, i wouldn't really suggest a Maya - i'd get one from Jan at Sleeping Baby Productions. her prices are probably about half what Maya Wrap charges and her shoulder design is soooo much more comfy. there are lots of very nice ring slings on the market, but her prices are just phenomenal. if you want something super pretty and have money to spend, there are tons of options that'll make you happier than a maya wrap. if you do get a maya, make sure you get the lightly padded one.
but my favorite birth to toddlerhood carrier hands-down is a mei tai. you can't go wrong with something like a Kozy or a babyhawk, though there are many great mei tai's on the market beyond those, too.
hope that helped a little
