This is not the opportunity to whip out a precious bottle of vintage claret - a bottle of wine at the modest end of the spectrum will suffice. Basically, if you are willing to drink it, cook with it - that's my approach anyhow. Too cheap can result in wine being a lot weaker, thinner, and more acidic which will impact the flavour of the dish.
As an alternative to chestnut mushrooms, why not try shiitake? I have used on a couple of occasions and works a treat.
Finally, do not worry if the whole casserole cannot be devoured on the first sitting, it reheats just fine and some would argue that it tastes even better the second time around. Gently simmering in a pan until piping hot instead of a zap in the microwave is always preferred.