I would use the plaid so that the appliqued vehicles were more prominent in the quilt. Might use the last fabric with the vehicles as the backing fabric. As I understand your post, you usually applique by hand. When you applique by machine you do not have to tie off the threads, only back stitch or stitch in place. Much easier and the threads are more secure than in tying off, also goes so much faster. Your blocks are lovely. If you sandwich the quilt first, then the applique stitch can be your quilting stitch, just select the batting so that it doesn't need close quilting, or just fleece or minky as a backing.