First, pre-seasoned dutch ovens are much more common these days than they used to be. In fact, I'm not sure Lodge sells them otherwise anymore. Definitely go with the Lodge brand IMO.

Second, the number of coals really depends on what you're cooking but a good rule of thumb that I was told was for the bottom subtract 2 from the size of the DO (eg 12" oven = 10 coals on bottom) and add 2 to the size for the top (eg 12" oven = 14 coals on top) which should give you approx. 350 degrees inside. Don't know how accurate that is for sure but it's always worked well for me.

Last, stacking ovens is a great way to cook. Put you main course on the bottom, then potatoes (or like food) in the middle and add dessert to the top. You'll use less coals and your food cooks really well that way. You just have to adjust for varying cooking times and possibly temps. If you really want to get into it, check out a Volcano. They're really great for DO cooking too.