First of all, congratulations. You got a great bike at a great price.
As for tires I would try some 32's and see how you like them. They seem to be a favorite around here, give some more traction (depending on tread) and stability, take out some of the bumps and jars that you would take with skinner or higher pressure tires yet still let you zip.
A cyclocross bike is not a mountain bike and there are very different feels. There are some resources for self-fitting a bike such as sheldon brown's (which I can't find right now) or see if your LBS has a good reputation for doing fits and how much they charge.
I am still learning about gearing so this is what I know. A double and triple "crank" is the number of rings/cogs attached to your pedals. This system (cogs and arms) is the crank. Double cranks have become more popular as far as I know in recent years, although they have been around a long time. I have a double crank with the large cog having 50 teeth and the smaller has 34 teeth (50/34 is the short hand). This is also called a compact double and why that is I don't exactly know.
My rear cassette has cogs ranging from 11-28 teeth. I do a lot on my bike, its a cyclocross like yours. I commute 15-20 miles roundtrip with hilly terrain. I ride some singletrack and go up muddy inclines and over roots. I feel confident that I could go on a weekend tour with this system as well and plan to by the end of the summer. So for what its worth there it is.
I have been told that changing either my crank or rear cassette might benefit me in some conditions, what those conditions exactly are I'm not sure yet as the bike has done everything I have wanted it to do and well.
As for your additions of "Brooks B17, MKS Touring Platform Pedals, and I already have Topeak Explorer rack" I think those are great and you can't go wrong with what you have chosen.
I suggest either finding some trusted on-line or local knowledge and getting your fit dialed in. Then ride the bike a lot, then see what you want to start swapping and changing out besides your ideas for a saddle and pedals. I've found that things I initially thought I wanted to change are not what I want now after riding my bike 500 miles or so. Pay particular attention to pain! Fix those problems first.