@ Jennifer
On the subject of a person tossing in his/her sleep and the advantage of a quilt vs bag, I think it all depends on the person’s ability to learn to manage the small drafts. I turn a lot at night from left to right to back and then start all over again. I also sleep with two pillows: one under my head and another between my knees when I am on my side. I transitioned from a bag to a quilt as the last step on my journey toward lightness. Since I camp a lot on the Pacific Coast Range where there is a lot of rain and fog and I was worried about the expense I got a synthetic quilt from MLD.
The best way to check girth would be go into your local backpacking store with a measuring tape and have the person lie in the bag and try tossing and turning. Measure the total girth of the bag that seems to have enough room. In my opinion 1/2 of that total, should be about what you need for a quilt. You want enough to tuck around your body or down the sides of your pad. There is a contrary view that that the wider the quilt is the more drafts you might get.
Many people have had great experience with the EE line of synthetic quilts that are sold at a good price point. Alan Dixon who used to write a lot for BPL did a roundup of lower cost quilt offerings even with 800 down here:Â http://www.adventurealan.com/best-lightweight-backpacking-quilts-sleeping-bag/
He also has an article on cheap, light backpacking. Ryan Jordan also published in BPL an article and lightweight gear you could buy at REI.
You might want to make the quilt the last purchase, The problem with a cheap bag is that they are heavy although Marmot and Mountain HW are bringing out new synthetic bags that are lighter.
At the end of summer, Big Agnes and other vendors like Marmot and Mtn HW put bags on sale. There is also the used down bag route. You occasionally see the Western Mountaineering or Marmot Hydogen bags sold used here which are 800 down or higher. You could wash it yourself or have it professionally cleaned by Rainy Pass Repair. I and over 6 feet and 215 lbs and found the WM bags too constricting.