That's a very nice sampler. It would be a difficult quilt to add sashing to, though, because the blocks are different sizes.
If I was to add sashing, I would use a plain white. The sashing would give some visual breaks between the blocks, which would make it less busy (and easier to admire the individual blocks). You would have to rearrange some of the blocks to accommodate this. I would try laying out your blocks on a sheet of white fabric, leaving spaces like sashing, and seeing if you like the look.
You may want some to be sewn together without sashing, but you may not... see what you think as you experiment! You might want to un-join the ones you've sewn together already, depending which it is. For example, there is a set of three pinwheels that could be joined, or not, depending on the look you like.
For backing, I would want to use something red and white with a large-scale print, like the Rhapsody wide back in red from Connecting Threads. (
http://www.connectingthreads.com/Fab...L60020326.html)