the batting will not be too thick. since you trim away that little bit from the edge the next set of blocks gets sewn to, it all comes together and doesn't overlap.
if you don't trust yourself, practice a little with some sample blocks. you'll have the hang of it in no time. trust me. if i can do it, so can you.
the white muslin shouldn't present any problems but unless you know it won't shrink more than the fabrics on the front, i do recommend you prewash it and throw it into a high heat dryer to preshrink.
and if you decide to use patterned fabrics for the back ... who says they have to match the front? who says the back has to be made from only one fabric? have some fun, girl. LOL
as long as you use colors on the back that will not clash too badly with the binding, it'll be just fine. reversible. 2 quilts fer the work of 1 (a 2-fer).
the good feeling you get when you've proved to yourself that you can do it will make a very nice "payment", thank you.
:P