Great quilt! I'd do a plain border, picking out one of the colours you like from the blocks. Alternatively, if you want to keep the celestial theme, dark navy.
SITD is fine for quilting, but a lot of quilters find it very difficult to stay precisely in the 'ditch'. Some people find it easier to do a wavy line which criss-crosses their lines. Try a small sample first, to see what you prefer and how easy you find it.
As for batting, it's a matter of personal preference/budget. The most luxurious (ergo expensive!) is silk, followed by wool, cotton or bamboo and finally, polyester.
Re batting - I prefer 100% cotton Quilter's Dream myself (there are 3 weights - Request, Select and Deluxe) or a bamboo, bamboo/cotton blend. Warm and Natural is very popular, but be careful you lay the correct side against your top or you may get 'bearding', especially on the darker bits. Happy quilting!
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