I love cute longies! As to your questions, first, the smell factor. Wool has natural antibacterial properties. Over time, they will start to smell, but not for a week or two, depending on how often you use them. That is a sign that it is time to wash them! They can also hold 30% their weight in moisture and wick it away from baby allowing their skin to breathe. They are great covers when baby has a rash and you want them to get air circulating (unlike PUL) Most longies are knit from minimally processed wool which has not had it's lanolin stripped from it. This makes them softer than a wool sweater that you may have that was knit from processed wool in a factory. There are also different kinds of wool that are much softer than others. Merino, blue faced leicester, targhee, are all very soft, very absorbent wools. Also, the more you wash wool in a lanolin rich wool soap, the softer it gets.
As for the onesie issue, we just very infrequently wear them. I have lots of short and long sleeved t's - some embroidered, painted or tyedyed. I have too many pairs of longies for my one babe - but it means I wash less and have the luxury of variety! The snaps in the crotch of things bug me. I also find that a lot of mainstream clothing doesn't fit well over a big diaper bum, so that is another great thing about longies. They can be pricey, but I have a lot of gn pairs that dd wore, that Booker is wearing now. And he is still in the newborn/small longies - pretty good for 5 months old and 15 pounds!