You'd be surprised at what temps you'd begin to wish you had something under you. I'd guess in the 60"s. Tho a lot may depend on site and set up.
UQ's are optimal tho there are ways around it. Do you have a sleeping pad of any sort?
I would suggest you beg and borrow what you can to get yourself out and save your pennies.
also, while the WBBB is a great hammock, if you're juggling funds, you can put that on the back burner too and get something that will get you out and off the ground for less. Used gear is also a good option. Lots of excellent finds can be had on the site.
FWIW, I use a 20* plus 1oz HG phoenix. Combined with a pad, and appropriate bag, I've gotten into the single digits and have been fine.