You should consider getting the headset from your LBS. Unless you are going to install it yourself with a DIY tool, you probably won't save enough on the Internet to make up the difference in shipping+labor. I built my Cross Check with a Cane Creek S3 headset and that has been very solid at a low price. Unfortunately, Cane Creek has changed their naming scheme, so I don't know what the current equivalent is.

You should also see if the LBS will give you a loaner stem until you dial in your fit and cut the steerer. Again, buying the stem from them will grease that process.

Some say you should have the bottom bracket faced before installed the crank.

Something like a handlebar you ought to be able to find cheap or free by asking around in the local cycling community. I know I usually have one on the shelf that I don't intend to use.

If you can avoid it, I wouldn't go cheap on the cables. The Jagwire Racer set is nice and performs well.

The Tektro CR720's are definitely the best for the price.

A cheap seatpost is generally close to the cost of shipping, so the LBS might be competitive there too if you aren't sourcing it from a discounter with other stuff. You need 27.2 for the Cross Check. Kalloy makes some cheap seat posts that work well enough until you have the budget for a Thomson. Sometimes you can find two-bolt Eastons online at closeout prices.