I just came back last week from the Viva Italia trip (read my trip report on that thread). I took just my DS15 and left DH and three girls at home! It was a wonderful trip. I, like you, knew I wanted to travel abroad but didn't think I could navigate a place like Italy by myself - London, Paris, maybe; but not Italy. These DIS boards actually sold me on using
ABD! I encourage you to read trip reports and ask lots of questions - all ABD travelers are very enthusiastic and are a wealth of information.
When we went, we had one room for the both of us - two beds in the room (granted, at times they were separated by only a few inches, but two beds nonetheless!). From what I understand, Viva Italia hits the true "Italian" spots...all the places you think of when you think of Italy. The Bella Italia spends time in the southern portion of Italy, which is very beautiful. Several people in our tour spent time down there before doing our tour and they thought it was beautiful, but didn't think they would enjoy that week-long tour, even though it does travel to different places. As for the heat, we went in mid-June and it was hot; Rome was hot, Florence was brutal, Artimino was wonderful, and Venice was lovely. If I had to do it again, I would maybe shoot for even earlier in June or late May if you do not like heat. We are from Florida so are used to heat, but Florence was very uncomfortable.
We booked through Costco and yes, you do have to be a member. Membership is, I believe, $25 for the year, and if you book through them, you get a $300 gift card (at least that was what they gave me at the time I booked), that you will receive upon returning from your trip. Other than that incentive, there is no real reason to book through them as when you call, you're immediately directed to an ABD desk.
I highly recommend ABD and I hope you'll be able to enjoy a vacation with them!