“(I know - you get what you pay for, but I'm staying positive for now).”
While I have stayed in some truly great hotels and B&Bs over the years, the absolute best accommodations I have ever enjoyed on many levels have all been in apartments/condos via airbnb. You certainly don’t have anything to be worried about in my opinion. I use airbnb almost exclusively now and have never, ever had a bad experience. As for your gift idea, it’s not really necessary unless you actually meet your host in person and wouldn’t mind getting to know him/her a little bit better. And in that case, I would suggest meeting for coffee sometime before you leave. You could even give them your small gift box of chocolates at that time if you think it’s appropriate. I have never offered or left a gift, but I have gotten to know some hosts, and there are some really cool people hosting with airbnb. Enjoy the experience!
I almost neglected to mention that, just like any other lodging, a great rate doesn’t necessarily mean a great place. Ensure the place is in a part of town that’s okay for you, and read the reviews. Not only will the place have reviews, but the hosts themselves will often have reviews from other hosts (a lot of airbnb hosts travel and use airbnb themselves). When you get back, take the time to submit a review, and expect a review of you from the host. It all helps the airbnb community in a good way!