Most of what you ask is just a matter of personal opinion. So, here's mine, lol.
1. I think your quilt top is pretty. Understated, yes, but that makes it more elegant.
2. Quilt shops and on-line stores will continue to have sales, so you don't need to be in a hurry to buy your backing fabric. Just when ever you get the fancy. More a matter of finding what you want, then buying it. At least that's my M.O.
3. I would go for some contrast, but not shocking contrast. An all over print will hide any quilting flubs, if you use a thread color that blends in. Or if you have enough of the fabrics you are using in the top, you could use them for the backing. Just cut panels of each fabric and sew together. Or use one of the fabrics with a middle band of another color. It's a fun surprise for the quilt recipient.
4. Don't sell yourself short, and don't "dis" yourself when showing your quilt to others. You are doing a great job. One of the lessons we might hope to learn is to make use of what we have. You are learning that.